OSAC 2022-S-0024 Best Practice Recommendations

for Evaluative Forensic DNA Testimony

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Best Practice 

Recommendations for 

Evaluative Forensic DNA 

Testimony

Human Forensic Biology Subcommittee

Biology Scientific Area Committee

Organization of Scientific Area Committees (OSAC) for Forensic Science
OSAC 2022-S-0024 Best Practice Recommendations

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Draft OSAC Proposed Standard

OSAC 2022-S-0024

Best Practice Recommendations for 

Evaluative Forensic DNA Testimony

Prepared by 

Human Forensic Biology Subcommittee

Version: 1.0

January 2022

Disclaimer:

This OSAC Proposed Standard was written by the (subcommittee) of the Organization of Scientific Area 

Committees (OSAC) for Forensic Science following a process that includes an open comment period. 

This Proposed Standard will be submitted to a standards developing organization and is subject to 

change.

There may be references in an OSAC Proposed Standard to other publications under development by 

OSAC. The information in the Proposed Standard, and underlying concepts and methodologies, may be 

used by the forensic-science community before the completion of such companion publications.

Any identification of commercial equipment, instruments, or materials in the Proposed Standard is not a 

recommendation or endorsement by the U.S. Government and does not imply that the equipment, 

instruments, or materials are necessarily the best available for the purpose.

To be placed on the OSAC Registry, certain types of standards first must be reviewed by a Scientific and 

Technical Review Panel (STRP). The STRP process is vital to OSAC’s mission of generating and 

recognizing scientifically sound standards for producing and interpreting forensic science results. The 

STRP shall provide critical and knowledgeable reviews of draft standards or of proposed revisions of 

standards previously published by standards developing organizations (SDOs) to ensure that the 

published methods that practitioners employ are scientifically valid, and the resulting claims are 

trustworthy.

The STRP panel will consist of an independent and diverse panel, including subject matter experts, 

human factors scientists, quality assurance personnel, and legal experts, which will be tasked with 

evaluating the proposed standard based on a comprehensive list of science-based criteria.

For more information about this important process, please visit our website 

at: https://www.nist.gov/topics/organization-scientific-area-committees-forensic-science/scientifictechnical-review-panels. 
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1 Foreword

2 This document provides guidelines for best testimony practices in Forensic Biology. Its aim is to present 

3 recommendations based on a recognized framework for casework assessment and interpretation (CAI) 

4 [Cook et al., 1998 CAI], which ensures the following desired qualities [ENFSI Guidelines 2015, Assoc. 

5 For. Sci. Providers 2009]:

6 ● Balance: The consideration of at least two propositions on the key issues and establishing the 

7 relevant background information as it relates to each proposition, as understood by the expert, 

8 avoids the evaluation to be one-sided (i.e., only taking into account one party’s views) or biased.

9 ● Logic: The assessment of the probability of the evidence given each proposition and the relevant 

10 background information prevents the expert from making statements about the probabilities of the 

11 propositions which would be usurping the role of the judge or jury.

12 ● Robustness: The use of sound knowledge, accepted tests and examinations, verified databases, 

13 and applicable published studies increases the quality of the expert's testimony.

14 ● Transparency: The application of a recognized framework in the expert’s case notes will show 

15 the foundation and thought process used so that the expert will be able to demonstrate how the 

16 opinion was developed.

17 This framework promotes a logical approach for evaluating the forensic scientist’s findings, and is based 

18 on the Four Principles of Evidence Evaluation [Evett and Weir, 1998, Evett 2015]:

19 1. To evaluate the uncertainty of any given proposition, it is necessary to also consider at least one 

20 alternate proposition (i.e., propositions are dealt with in pairs).

21 2. Scientific evaluation of the evidence is based on the question ‘What is the probability of 

22 observing the evidence if the proposition were true?’ for each of the competing propositions. In 

23 mathematical terms, this is written as Pr(E|H), where Pr is “probability”, E is “evidence”, | is 

24 “given”, and H is the proposition.

25 3. Scientific evaluation of the evidence depends not only on each of the competing propositions, but 

26 also on the framework of case circumstances within which the evidence is to be evaluated. This 

27 expands the probabilities to Pr(E|H,I), where I is the relevant case-specific information.

28 It is logically meaningless to suggest that any evidence has value in itself as support for any particular 

29 proposition in isolation. Its value depends entirely upon its ability to discriminate between one 

30 proposition and an alternative proposition. This leads us to the fourth principle [Evett 2015]:

31 4. The value of the evidence is the ratio of two probabilities of the evidence given each of two 

32 alternate propositions within the case context. Mathematically, this will read:

33

34 and is termed the Likelihood Ratio or LR.
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35 The scientist is the best person to help the jury understand the evidence by performing this task, given 

36 their insight into the issues at hand. Transferring these responsibilities to the jury or Court increases the 

37 risks of misunderstanding and for inappropriate conclusions to be made. 

38 Application of the approach in the witness box requires a working knowledge of the four principles and a 

39 robust and educated response to questions so that the value of the evidence is conveyed in a balanced, fair 

40 and meaningful way, and inappropriate questions, such as “Is XX possible?”, are effectively rebutted. 

41 Any comment on XX by the expert is a comment on the ultimate issue, and that is reserved for the jury.

42 This document is organized according to the Hierarchy of Propositions framework, which consists of the 

43 following levels: sub-sub-source, sub-source, source, activity, and crime or offence level [Cook et al., 

44 1998 hierarchy, Evett et al., 2002, Gittelson et al., 2016]. As the expert’s evaluation moves up the 

45 hierarchy of propositions (e.g., from sub-source to activity), the expert is communicating more expert 

46 knowledge of relevance to the court. In order to do this effectively, the scientist will incorporate data or 

47 personal knowledge, for example regarding body fluid attribution, DNA transfer and persistence and 

48 background levels of DNA as well as contamination—different levels in the hierarchy require considering 

49 separate factors and data. It is appropriate for the scientists to assist, where possible, in this way because 

50 the decision maker at court (jury or sometimes judge) does not have this specialized knowledge. 

51 A high priority for this subcommittee was to include explicit guidance for testimony with regard to 

52 activity level propositions; that is, testimony that considers factors such as DNA transfer, persistence, and 

53 recovery. This is an area for which no real guidance currently exists in the United States. In cases where 

54 the activity is the issue, scientists will assess the value of the results (i.e., assign the probability of the 

55 results) given at least two competing propositions. We recognize that best practice would be to include 

56 the evaluation of DNA evidence given activity level propositions in the initial laboratory report. However, 

57 currently very few laboratories are doing this, yet questions regarding activity level propositions show up

58 routinely in trials. The goal of this document is to address this immediate need for testimony guidance.

59 This document will not address offence level propositions in any fashion, as it is rare for forensic 

60 scientists to add value by the use of their specialized knowledge when considering this level of 

61 propositions. 

62 The Recommendations (Section 4) are presented in four subsections: 

63 4.1.Communication of results given sub-sub-source level propositions

64 4.2.Communication of results given sub-source level propositions

65 4.3.Communication of results given source level propositions

66 4.4.Communication of results given activity level propositions.

67 Each of these subsections was written to provide a complete, standalone list of recommendations for the 

68 given level, so that each one can be read by itself without reference to the other subsections.

69 The adherence to these guidelines will ensure that the forensic scientist’s testimony is logically and 

70 scientifically sound without usurping the role of the judge or jury. Adopting them will improve the 

71 quality, accuracy and consistency of communicating results across practitioners, and lead to an increase in 

72 quality by providing added value to the Court. The document provides the theoretical background behind 
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73 the approach, real world guidelines for best testimony practices, and some working examples of how this 

74 might be applied. The practice could also be applied to statement writing.

75 All hyperlinks and web addresses shown in this document are current as of the publication date of this 

76 standard.

77

78 Keywords: forensic DNA testimony, communication, likelihood ratio, activity level, hierarchy 

79 of propositions

80
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81 Table of Contents 

82 1 Scope............................................................................................................................................................ 

83 2 Normative References.................................................................................................................................. 

84 3 Terms and Definitions................................................................................................................................... 

85 4 Recommendations.........................................................................................................................................

86 Annex A (informative) Supporting Information............................................................................................... 

87 Annex B (informative) Examples................................................................................................................ 

88 Annex C (informative) Bibliography................................................................................................................

89
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90 1 Scope

91 This document describes best practices for the communication of biological results and opinions in the 

92 field of Forensic Biology. It addresses testimony given sub-sub-source, sub-source, source, and activity 

93 level propositions. This document will not consider the highest level of the Hierarchy of Propositions 

94 (crime level). 

95

96 This document does not give guidance on how to obtain findings (DNA results/statistics) or form 

97 opinions (how or when to have an opinion that evidence supports a proposition or that the findings are 

98 neutral for a given pair of propositions). Rather, the purpose is to focus on best practices for the 

99 communication of findings and opinions in the role of an expert witness.

100

101 2 Normative References

102

103 1. ENFSI European Network of Forensic Science Institutes (2015). ENFSI Guideline for Evaluative 

104 Reporting in Forensic Science, available at: http://enfsi.eu/wp105 content/uploads/2016/09/m1_guideline.pdf.

106 2. P. Gill,* T. Hicks*, J. Butler, E. Connolly, B. Kokshoorn, L. Gusmão, N. Morling, R. van Oorschot, 

107 W. Parson, M. Prinz, P. Schneider, T. Sijen, D. Taylor. DNA Commission of the International 

108 Society for Forensic Genetics: Assessing the value of forensic biological evidence - guidelines 

109 highlighting the importance of propositions Part II: Evaluation of biological results given activity 

110 level propositions. Forensic Science International: Genetics 44 (2020) 102186.

111 3. Robertson B, Vignaux GA, Berger CEH, Interpreting Evidence: Evaluating Forensic Science in 

112 the Courtroom, 2nd Ed. John Wiley & Sons, Ltd; 2016.

113 4. Cook R, Evett IW, Jackson G, Jones PJ, Lambert JA. A model for case assessment and 

114 interpretation. Science & Justice 1998; 38(3): 151-156.

115 5. Cook R, Evett IW, Jackson G, Jones PJ, Lambert LA. A hierarchy of propositions: Deciding 

116 which level to address in casework, Science & Justice 38(4) (1998) 231-239.

117 6. Evett IW, Jackson G, Lambert JA. More on the hierarchy of propositions: Exploring the 

118 distinction between explanations and propositions. Science & Justice 40 (2000) 3-10. 

119 7. Evett IW, Gill PD, Jackson G, Whitaker J, Champod C. Interpreting small quantities of DNA: the 

120 hierarchy of propositions and the use of Bayesian networks, Journal of Forensic Sciences 47(3) 

121 (2002) 520-530.

122 8. Gittelson S, Kalafut T, Myers S, Taylor D, Hicks T, Taroni F, Evett IW, Bright J-A, Buckleton J. 

123 A Practical Guide for the Formulation of Propositions in the Bayesian Approach to DNA 

124 Evidence Interpretation in an Adversarial Environment, Journal of Forensic Sciences 61(1) 

125 (2016) 186-195.

126 9. Evett IW, Weir B. Interpreting DNA Evidence, Sinauer Associates, Sunderland MA, 1998.

127 10. I.W. Evett, The logical foundations of forensic science: Towards reliable knowledge, 

128 Philosophical Transactions of the Royal Society of London B 370 (2015) 20140263.
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129 11. Association of Forensic Science Providers. Standards for the formulation of evaluative forensic 

130 science expert opinion, Science & Justice 49 (2009) 161-164.

131 12. C.G.G. Aitken, P. Roberts, G. Jackson. Fundamentals of Probability and Statistical Evidence in 

132 Criminal Proceedings: Guidance for Judges, Lawyers, Forensic Scientists and Expert Witnesses, 

133 Practitioner Guide No. 1. Royal Statistical Society, 2010.

134 13. National Commission on Forensic Science. Ensuring That Forensic Analysis Is Based Upon 

135 Task-Relevant Information. Human Factors Subcommittee, Department of Justice and National 

136 Institute of Standards and Technology, 2015.

137 14. I.W. Evett, Avoiding the transposed conditional, Science & Justice 35 (1995) 127-131.

138 15. Taroni F, Garbolino P, Biedermann A, Aitken C, Bozza S. Reconciliation of subjective 

139 probabilities and frequencies in forensic science. Law, probability and Risk. 2018;17(3):243-262.

140

141 16. W.C. Thompson, E.L. Schumann, Interpretation of Statistical Evidence in Criminal Trials: The 

142 Prosecutor's Fallacy and the Defense Attorney's Fallacy, Law and Human Behaviour 11 (1987) 

143 167-187.

144

145 3 Terms and Definitions

146

147 Bayesian/logical framework 

148 A framework based on (the odds form of) Bayes’ Theorem, which is a mathematical idealization of the 

149 belief about a set of propositions is updated based on the (value of the) evidence. This is a framework 

150 used to evaluate evidence or results given two different propositions summarizing the point of view of the 

151 parties as understood in the case. The value of the evidence is given by the expert in the form of a 

152 Likelihood Ratio (LR). The focus of the LR is always on the evidence, not on the proposition. The value 

153 of evidence (LR) presented by the expert can then be combined with the other non-expert evidence to 

154 update the decision maker’s beliefs in the proposition. The odds form of Bayes’ theorem is:

155

Pr(H1|E,I)

Pr(H2|E,I)

=

Pr(E|H1,I)

Pr(E|H2,I)

×

Pr(H1| I)

Pr(H2| I)

156

157

158

159

160

161 Bayesian Networks

162 Bayesian Networks are a marriage between graph theory and probability theory. They provide a tool for 

163 describing (potentially complex) dependencies between variables whose value is uncertain; thus may be 

164 used to calculate an LR given activity level propositions. 

165

166 Case Assessment and Interpretation (CAI) model

167 The CAI model is a framework for structuring forensic examinations and reporting results. It allows 

168 scientists to formulate examination strategies, record their expectations of the probability of various 

169 outcomes prior to analysis, and then compare the expected and observed results. The process of assigning 

170 probabilities given the activities before knowing the results ensures that scientists are not influenced by 

171 what they have obtained (i.e., avoiding post hoc rationalization).

Posterior odds Likelihood Ratio Prior odds
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172

173 Explanation

174 In the context of a forensic science evaluation, explanations are generated after the forensic findings have 

175 been obtained. While an explanation has the potential to account for particular observations, it does not 

176 qualify as a formal proposition because - often - it may be a statement of the obvious, speculative, or 

177 fanciful. 

178

179 Hierarchy of propositions

180 The Hierarchy of propositions is a framework to help address different questions of relevance to the trier 

181 of fact. In the context of criminal proceedings, propositions can be classified into broad categories (or, 

182 hierarchical levels):

● ‘sub-sub-source level’1 183 for propositions about the source (i.e., person) of a part (i.e., major or

184 minor contribution) of the DNA profile.

● ‘sub-source level’2 185 for propositions about the source (i.e., person) of the DNA

186 ● ‘source level’ for propositions about the source (i.e., person or item) from which the trace 

187 material originated (e.g., propositions addressing who is the source of the body fluid)

188 ● ‘activity level’ for propositions about an activity or a happening (e.g., propositions addressing 

189 how or when the DNA was transferred)

190 ● ‘offence level’ for propositions that refer to the commission of a criminal offence

191

192 Likelihood ratio (LR)

193 In the context of evidence interpretation: The probability of the evidence under one proposition divided 

194 by the probability of the evidence under an alternative, mutually exclusive proposition. The magnitude of 

195 its value expresses the weight of the evidence.

196

197 Note: The likelihood ratio can be numerical or qualitative (especially for LR given activity level 

198 propositions). While the individual probabilities associated with the evidence given each proposition are 

199 useful, the full value of the LR comes from comparing the probabilities of the results given different 

200 propositions, because the value of the results depends entirely upon its ability to discriminate between one 

201 proposition and another.

202

203 Nature of the biological fluid

204 The question regarding the type of material (e.g., blood, semen, saliva) will be defined as questions about 

205 the nature of the material. Source questions will be defined as questions regarding from which person the 

206 body material/DNA came from.

207

208 Possible

209 Any non-zero probability. There is no distinction between high or low probability when using possible in 

210 relation to opinion testimony. The fact that something (i.e. the evidentiary result) is “possible” is not 

211 useful in determining whether or not the evidence supports one proposition or another. It is always 

212 desirable to put things in terms of probability, and best to compare probabilities of the findings given two 

213 competing propositions.

214

215 Posterior probability

216 In the Bayesian framework, this is the jury’s probability of the propositions taking into account all the 

217 elements of the case, including the forensic results presented by the expert (e.g., the DNA evidence).

218

1

sub-sub-source only has relevance or meaning in the interpretation of DNA mixture evidence.

2

sub-source only has relevance or meaning in the interpretation of DNA evidence.
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219 Prior probability 

220 In the Bayesian framework, this is the jury’s probability of the propositions taking into account all the 

221 elements of the case, excluding the forensic results presented by the expert (e.g., the DNA evidence).

222

223 Probability

224 Probability is a measurement device for uncertainty. In the Bayesian framework, it serves the purpose of 

225 expressing an individual’s personal degree of beliefs about uncertain propositions conditional on the 

226 status of information of the subject who assesses it. Probability informed by relevant/appropriate data 

227 (e.g., scientific publications, in-house experiments, experience, or knowledge). Probability is governed by 

228 several axiomatic laws that constitute a fundamental framework for inductive logic. There are several 

229 schools of thought on the definition of probability; however, it is important to understand that the laws 

230 of probabilities hold, regardless of the definition.

231

232 Propositions

233 Propositions are statements that are either true or false, and that can be affirmed or denied. There is no 

234 requirement that any proposition must be “true” for assigning an LR. Propositions should be formulated 

235 in pairs (e.g., views put forward by the parties to the cases) given a background of task-relevant case 

236 information and assumptions. Propositions should be amenable to a reasoned assignment of credibility by 

237 a judicial body. A basic criterion for propositions is that they should be formulated in such a way that it is 

238 reasonable for the scientist to address a question of the form: ‘what is the probability of the observations 

239 given this proposition within the framework of circumstances?’

240 Propositions should be distinguished from explanations that do not have the aforementioned properties. 

241 Propositions should be based on case information and the issue where forensic science can help, not on 

242 the biological results. 

243

244 Transfer and persistence

245 Transfer is the mechanism that allows the movement of evidence (e.g. DNA) from its origin (source) to 

246 wherever it is recovered in the context of the case (e.g., primary, secondary).

247 Persistence pertains to the ability of the material to remain where it was deposited prior to its recovery by 

248 the forensic scientist. However, often the probability of recovery given the evidence was transferred and

249 persisted is of more relevance. This is critical for any discussion in the context of activity level of 

250 propositions. 

251

252 Transposed conditional

253 In forensic science this often refers to any statement from the scientist about the probability of the 

254 proposition given the evidence (incorrect) rather than the probability of the evidence given the proposition 

255 (correct). Formally, this is stating the probability of A given B (i.e., Pr(A|B)) as the probability of B given 

256 A (i.e., Pr(B|A)), or vice versa. This is commonly referred to as the “prosecutor’s fallacy”. 

257

258 4. Recommendations

259

260 4.1 Communication of results given sub-sub-source level propositions 

261 4.1.2. The expert should not communicate at this level if the person who is the source of this DNA is not 

262 the issue of interest to the court or/and if the issue is the activity that led to the DNA.

263 4.1.3 The use of the terms specific to this level, such as “major contributor” and “minor contributor”, 

264 should be defined by laboratory protocols.
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265 4.1.4 It is recommended to reserve the use of “major” and “minor” terms for cases where the issue of 

266 interest may move to the activity level. 

267 4.1.5 The expert should follow the four principles of evidence evaluation [Evett and Weir 1998, Evett 

268 2015]: 

269 4.1.5.1 The expert should take into consideration at least one alternative proposition, so that the 

270 evaluation considers a pair of propositions. 

271 4.1.5.2 The expert should only make statements about the probability of the evidence given a proposition 

272 [Thompson and Schumann, 1987]. The expert must avoid making any statement about the probability of a 

273 proposition given the evidence (i.e., transposing the conditional) [Aitken et al., 2010, ENFSI Guidelines 

274 2015, Evett 1995, Robertson et al., 2016, Evett and Weir, 1998, Evett 2015, Assoc. For. Sci. Providers 

275 2009]. Equally, the expert must avoid giving any statement on the probabilities of the propositions 

276 themselves. 

277 4.1.5.2.1 The expert should be aware that most hypothetical questions are asking for an opinion about the 

278 proposition (or explanation), not about the evidence.

279 4.1.5.2.2 The expert must avoid transposing a conditional when answering direct questions from one of 

280 the parties as well as when considering both propositions. 

281 4.1.5.3 The evaluation should take place in a framework of task-relevant case circumstances. This means 

282 that the probabilities are conditional upon the information and data specific to the specimen/case [ENFSI 

283 Guidelines 2015, Evett and Weir 1998, Evett 2015, Robertson et al., 2016, Gill et al., 2020].

284 4.1.5.3.1 Information should be directly task-relevant to the evaluation of the evidence [Evett and Weir, 

285 1998, Evett 2015, National Commission on Forensic Science]. 

286 4.1.5.3.2 The information relied upon to form the opinion of the expert should be disclosed and clearly 

287 communicated.

288 4.1.5.4. Probabilities of the evidence given a proposition should be evaluated in pairs (i.e., the probability 

289 of the evidence given the main proposition and the probability of the evidence given the alternative 

290 proposition) in order to give a likelihood ratio (LR). This ratio is the value of the forensic results and 

291 expresses the strength of the evidence [ENFSI Guidelines 2015, Evett and Weir 1998, Gill et al., 2020, 

292 Evett 2015, Robertson et al., 2016, Aitken et al., 2010].

293 4.1.6 Probabilities and ratios of probabilities should be used when communicating the value of the 

294 evidence [ENFSI Guidelines 2015, Evett and Weir 1998, Assoc. For. Sci. Providers 2009, Aitken et al., 

295 2010, Evett 2015, Robertson et al., 2016, Gill et al., 2020].

296 4.1.6.1 These probabilities may be explicitly calculated (e.g., published model for genotype probabilities) 

297 or they may be assigned notional (personal) probabilities based on published research data, data derived 

298 from internal laboratory studies and/or from casework experience (e.g., any published research data on 

299 transfer and persistence studies used to inform the expert opinion in a case) [Assoc. For. Sci. Providers 

300 2009, ENFSI Guidelines 2015, Taroni 2018].

301 4.1.6.2 The LR may be communicated numerically (1 million times more likely) or using a qualitative 

302 verbal statement (much more likely, somewhat more likely) after assignment of the numerical LR [ENFSI 

303 Guidelines 2015, Assoc. For. Sci. Providers 2009, Gill et al., 2020]. 
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304 4.2 Communication of results given sub-source level propositions 

305 4.2.1 The expert should use caution when communicating at this level if the POI agrees it is their DNA, 

306 such as in a question of consent, or if the source of the DNA is not disputed (as there is only one 

307 proposition and not two). If the person from whom the DNA originated is not contested, then there is no 

308 uncertainty on who is the source of the DNA, thus the calculation given sub-source level propositions is 

309 not meaningful. A sub-source statistic could easily be misunderstood in the context of the activities [Gill 

310 et al., 2020, ENFSI Guidelines 2015, Robertson et al., 2016]. 

311 4.2.2 The expert should follow the four principles of evidence evaluation [Evett and Weir 1998, Evett 

312 2015]: 

313 4.2.2.1 The expert should take into consideration at least one alternative proposition, so that the 

314 evaluation considers a pair of propositions. 

315 4.2.2.2 The expert should only make statements about the probability of the evidence given a proposition 

316 [Thompson and Schumann, 1987]. The expert must avoid making any statement about the probability of a 

317 proposition given the evidence (i.e., transposing the conditional) [Aitken et al., 2010, ENFSI Guidelines 

318 2015, Evett 1995, Robertson et al., 2016, Evett and Weir, 1998, Evett 2015, Assoc. For. Sci. Providers 

319 2009]. Equally, the expert must avoid giving any statement on the probabilities of the propositions 

320 themselves. 

321 4.2.2.2.1 The expert should not make any statement about the POI being the source of the DNA unless or 

322 until the source of the DNA is no longer in dispute. 

323 4.2.2.2.2 The expert should be aware that most hypothetical questions are asking for an opinion about the 

324 proposition (or explanation), not about the evidence.

325 4.2.2.2.3 The expert must avoid transposing a conditional when answering direct questions from one of 

326 the parties as well as when considering both propositions. 

327 4.2.2.3 The evaluation should take place in a framework of task-relevant case circumstances. This means 

328 that the probabilities are conditional upon the information and data specific to the specimen/case [ENFSI 

329 Guidelines 2015, Evett and Weir 1998, Evett 2015, Robertson et al., 2016, Gill et al., 2020].

330 4.2.2.3.1 Information should be directly task-relevant to the evaluation of the evidence [Evett and Weir, 

331 1998, Evett 2015, National Commission on Forensic Science]. 

332 4.2.2.3.2 The information relied upon to form the opinion of the expert should be disclosed and clearly 

333 communicated.

334 4.2.2.4. Probabilities of the evidence given a proposition should be evaluated in pairs (i.e., the probability 

335 of the evidence given the main proposition and the probability of the evidence given the alternative 

336 proposition) in order to give a likelihood ratio (LR). This ratio is the value of the forensic results and 

337 expresses the strength of the evidence [ENFSI Guidelines 2015, Evett and Weir 1998, Gill et al., 2020, 

338 Evett 2015, Robertson et al., 2016, Aitken et al., 2010].

339 4.2.3 Probabilities and ratios of probabilities should be used when communicating the value of the 

340 evidence [ENFSI Guidelines 2015, Evett and Weir 1998, Assoc. For. Sci. Providers 2009, Aitken et al., 

341 2010, Evett 2015, Robertson et al., 2016, Gill et al., 2020].
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342 4.2.3.1 These probabilities may be explicitly calculated (e.g., published model for genotype probabilities) 

343 or they may be assigned notional (personal) probabilities based on published research data, data derived 

344 from internal laboratory studies and/or from casework experience (e.g., any published research data on 

345 transfer and persistence studies used to inform the expert opinion in a case) [Assoc. For. Sci. Providers 

346 2009, ENFSI Guidelines 2015, Taroni 2018].

347 4.2.3.2 An RMP should be expressed as a probability. In practical terms, this probability communicates 

348 the rarity of an event.

349 4.2.3.3 The LR may be communicated numerically (1 million times more likely) or using a qualitative 

350 verbal statement (much more likely, somewhat more likely) after assignment of the numerical LR [ENFSI 

351 Guidelines 2015, Assoc. For. Sci. Providers 2009, Gill et al., 2020]. 

352 4.3 Communication of results given source level propositions 

353 4.3.1 These propositions are adequate if there is no issue about the nature of the biological material, (i.e., 

354 not disputed) or if the substrate is obvious (e.g., hair root, tooth, bone) [Evett et al., 2002, Gill et al., 

355 2020].

356 4.3.2 The assumption that the nature of the material is either known or not in dispute (e.g., the DNA came 

357 from the blood) and its scientific basis need to be disclosed. This should be framed as an expert opinion 

358 given the presumptive nature of many body fluid tests (e.g., serology, mRNA, appearance, case 

359 information).

360 4.3.3 When there is uncertainty about the nature of the biological material, no definitive statements should 

361 be used. In these circumstances it would be inappropriate to use source level propositions. Depending on 

362 the issue, sub-source or activity level propositions should be used.

363 4.3.3.1 Probabilities or ratios of probabilities may be used to communicate the serology findings. This is 

364 an independent exercise and will not alter the LR given sub-source level propositions. It would be a 

365 process by which the expert is able to convey their confidence regarding the likely biological material 

366 tested given all of the laboratory tests available to them.

367 4.3.3.1.1 The expert should be transparent in this opinion. When the expert has an opinion based on that 

368 “testing, the expert should assertively state this opinion, as this opinion will most likely be required to 

369 move to higher levels of the hierarchy of propositions.

370 4.3.3.2 When uncertainty precludes the expert from an opinion that distinguishes among body (fluid) 

371 sources, the expert may still answer inquiries about other levels of the hierarchy but must be transparent 

372 with the limitations due to the uncertainty in body source identification.

373 4.3.4 Depending on the case, when a mixed DNA profile is obtained, assigning a person’s DNA to a 

374 particular fluid may become difficult. Considerations must also be made for mixed biological material 

375 (e.g., semen and saliva).

376 4.3.5 If the nature of the biological material (i.e., specific biological fluid(s)) is of interest to the court, the 

377 expert should not communicate only the value of the DNA profile comparisons (sub-source level 

378 propositions). This avoids the translation of the LR given sub-source propositions to that of a higher level. 

379 It would be incorrect to apply an LR of 1 million given sub-source propositions to the LR consisting of 

380 the probability of the evidence if it is blood divided by the probability of the evidence if it is some other 

381 biological material.
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382 4.3.6 If the issue of interest to the court regards the nature of the biological material, activity level 

383 propositions are generally more suited as they also account for presence of this material for reasons 

384 unconnected to the facts in dispute.

385 4.3.7 The expert should follow the four principles of evidence evaluation [Evett and Weir 1998, Evett 

386 2015]: 

387

388 4.3.7.1 The expert should take into consideration at least one alternative proposition, so that the 

389 evaluation considers a pair of propositions. 

390 4.3.7.2 The expert should only make statements about the probability of the evidence given a proposition 

391 [Thompson and Schumann, 1987]. The expert must avoid making any statement about the probability of a 

392 proposition given the evidence (i.e., transposing the conditional) [Aitken et al., 2010, ENFSI Guidelines 

393 2015, Evett 1995, Robertson et al., 2016, Evett and Weir, 1998, Evett 2015, Assoc. For. Sci. Providers 

394 2009]. Equally, the expert must avoid giving any statement on the probabilities of the propositions 

395 themselves. 

396 4.3.7.2.1 The expert should be aware that most hypothetical questions are asking for an opinion about the 

397 proposition (or explanation), not about the evidence.

398 4.3.7.2.2 The expert must avoid transposing a conditional when answering direct questions from one of 

399 the parties as well as when considering both propositions. 

400 4.3.7.3 The evaluation should take place in a framework of task-relevant case circumstances. This means 

401 that the probabilities are conditional upon the information and data specific to the specimen/case [ENFSI 

402 Guidelines 2015, Evett and Weir 1998, Evett 2015, Robertson et al., 2016, Gill et al., 2020].

403 4.3.7.3.1 Information should be directly task-relevant to the evaluation of the evidence [Evett and Weir, 

404 1998, Evett 2015, National Commission on Forensic Science]. 

405 4.3.7.3.2 The information relied upon to form the opinion of the expert should be disclosed and clearly 

406 communicated.

407 4.3.7.4. Probabilities of the evidence given a proposition should be evaluated in pairs (i.e., the probability 

408 of the evidence given the main proposition and the probability of the evidence given the alternative 

409 proposition) in order to give a likelihood ratio (LR). This ratio is the value of the forensic results and 

410 expresses the strength of the evidence [ENFSI Guidelines 2015, Evett and Weir 1998, Gill et al., 2020, 

411 Evett 2015, Robertson et al., 2016, Aitken et al., 2010].

412 4.3.8 Probabilities and ratios of probabilities should be used when communicating the value of the 

413 evidence [ENFSI Guidelines 2015, Evett and Weir 1998, Assoc. For. Sci. Providers 2009, Aitken et al., 

414 2010, Evett 2015, Robertson et al., 2016, Gill et al., 2020].

415 4.3.8.1 These probabilities may be explicitly calculated (e.g., published model for genotype probabilities) 

416 or they may be assigned notional (personal) probabilities based on published research data, data derived 

417 from internal laboratory studies and/or from casework experience (e.g., any published research data on 

418 transfer and persistence studies used to inform the expert opinion in a case) [Assoc. For. Sci. Providers 

419 2009, ENFSI Guidelines 2015, Taroni 2018].
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420 4.3.8.2 The LR may be communicated numerically (1 million times more likely) or using a qualitative 

421 verbal statement (much more likely, somewhat more likely) after assignment of the numerical LR [ENFSI 

422 Guidelines 2015, Assoc. For. Sci. Providers 2009, Gill et al., 2020].

423 4.4 Communication of results given activity level propositions 

424 4.4.1 Laboratories should establish policies regarding testimony given activity level propositions when 

425 this is of interest to the court. It is best to have previously reported any relevant opinions in writing as 

426 well, however guidance on the determination and reporting of these opinions is outside the scope of this 

427 document. 

428 4.4.2 The expert should follow the four principles of evidence evaluation [Evett and Weir 1998, Evett 

429 2015]: 

430 4.4.2.1 The expert should take into consideration at least one alternative proposition, so that the 

431 evaluation considers a pair of propositions. 

432 4.4.2.1.1 Propositions should be based on case information and the issue where forensic science can help, 

433 not on the biological results [Evett et al., 2000, Gill et al., 2020].

434 4.4.2.1.2 Propositions are best formulated prior to the start of examination based on the information 

435 available at that time [Evett et al., 2000, Gittelson et al., 2016, Gill et al., 2020].

436 4.4.2.1.2.1 At a minimum, the expert should record their expectations of results if each of the propositions 

437 were true based on the available relevant case information.

438 4.4.2.1.2.2 Tools such as the CAI framework [Cook et al. (1998) CAI paper] and/or Bayesian networks 

439 may be useful in communicating the thought process that leads to the expert’s statement [Gill et al., 

440 2020]. 

441 4.4.2.1.3 The expert needs to be aware of the various transfer steps that may occur as a part of an activity 

442 proposition (e.g., primary, secondary). However, transfer in and of itself is not an activity, and the term 

443 “transfer” should not be a part of the proposition.

444 4.4.2.1.4 One proposition should align with the prosecution’s version of events.

445 4.4.2.1.5 The alternative proposition should represent an alternative version of events as understood by 

446 the expert.

447 4.4.2.1.5.1 In an adversarial judicial system, the defense is not required to offer any information or theory 

448 of the case. The expert should always keep this in mind, especially when the questions regard the activity 

449 that led to the presence of the DNA.

450 4.4.2.1.5.2 When no specific alternative is available from the defense, the expert should adopt a 

451 reasonable proxy proposition based on the case information in order to fully evaluate the evidence 

452 [Gittelson et al., 2016, ENFSI Guidelines 2015, Assoc. For. Sci. Providers 2009]. 

453 4.4.2.1.6 The expert should communicate that if a proposition(s) changes, the value of the evidence will 

454 change [ENFSI Guidelines 2015]. If the change of a proposition has a large effect on the LR, such as the 

455 evidence now has an LR of 1, it is crucial that the expert makes this known to the court.
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456 4.4.2.2 The expert should only make statements about the probability of the evidence given a proposition 

457 [Thompson and Schumann, 1987]. The expert must avoid making any statement about the probability of a 

458 proposition given the evidence (i.e., transposing the conditional) [Aitken et al., 2010, ENFSI Guidelines 

459 2015, Evett 1995, Robertson et al., 2016, Evett and Weir, 1998, Evett 2015, Assoc. For. Sci. Providers 

460 2009]. Equally, the expert must avoid giving any statement on the probabilities of the propositions 

461 themselves. 

462 4.4.2.2.1The expert should be aware that most hypothetical questions are asking for an opinion about the 

463 proposition (or explanation), not about the evidence.

464 4.4.2.2.2 The expert must avoid transposing a conditional when answering direct questions from one of 

465 the parties as well as when considering both propositions. 

466 4.4.2.3 The evaluation should take place in a framework of task-relevant case circumstances. This means 

467 that the probabilities are conditional upon the information and data specific to the specimen/case [ENFSI 

468 Guidelines 2015, Evett and Weir 1998, Evett 2015, Robertson et al., 2016, Gill et al., 2020].

469 4.4.2.3.1 Information should be directly task-relevant to the evaluation of the evidence [Evett and Weir, 

470 1998, Evett 2015, National Commission on Forensic Science]. In this case, this means that the expert 

471 should only consider information that is relevant to the evaluation of the evidence given activity level 

472 propositions.

473 4.4.2.3.2 The information relied upon to form the opinion of the expert should be disclosed and clearly 

474 communicated.

475 4.4.2.4. Probabilities of the evidence given a proposition should be evaluated in pairs (i.e., the probability 

476 of the evidence given the main proposition and the probability of the evidence given the alternative 

477 proposition) in order to give a likelihood ratio (LR). This ratio is the value of the forensic results and 

478 expresses the strength of the evidence [ENFSI Guidelines 2015, Evett and Weir 1998, Gill et al., 2020, 

479 Evett 2015, Robertson et al., 2016, Aitken et al., 2010].

480 4.4.3 Probabilities and ratios of probabilities should be used when communicating the value of the 

481 evidence [ENFSI Guidelines 2015, Evett and Weir 1998, Assoc. For. Sci. Providers 2009, Aitken et al., 

482 2010, Evett 2015, Robertson et al., 2016, Gill et al., 2020].

483 4.4.3.1 These probabilities may be explicitly calculated (e.g., published model for genotype probabilities) 

484 or they may be assigned notional (personal) probabilities based on published research data, data derived 

485 from internal laboratory studies and/or from casework experience (e.g., any published research data on 

486 transfer and persistence studies used to inform the expert opinion in a case) [Assoc. For. Sci. Providers 

487 2009, ENFSI Guidelines 2015, Taroni 2018].

488 4.4.3.2 The LR may be communicated numerically (1 million times more likely) or using a qualitative 

489 verbal statement (much more likely, somewhat more likely) after assignment of the numerical LR [ENFSI 

490 Guidelines 2015, Assoc. For. Sci. Providers 2009, Gill et al., 2020]. 

491 4.4.4 Hypothetical questions are often posed in court. The expert should consider/remember the following 

492 when choosing how to answer these questions.

493 4.4.4.1 A hypothetical question is often findings-based rather than case-information based. However, in 

494 the absence of any other proposition, it may be reasonable to adopt this scenario as one of the 

495 propositions for the likelihood ratio.
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496 4.4.4.2 If a hypothetical question is posed in the form of “Is this possible?”, the expert should 

497 communicate that the value of evidence is based on comparing probabilities of the results given 

498 competing propositions and not possibilities, and then give an answer in terms of relative probabilities. 

499 [Aitken et al., 2010, Evett 2015]. 

500 4.4.4.3 The expert should be cautious of hypothetical questions asking for the expert to provide an 

501 alternative explanation of the data (i.e., speculation) or making a comment on the probability of the 

502 hypothetical (proxy) proposition (i.e, transposing the conditional).

503 4.4.4.3.1 The expert must avoid giving any opinion on the probability of the activity. In this context, the 

504 term “activity” may include questions about direct/indirect transfer or about the same activities at 

505 different times [Gill et al., 2020].

506 4.4.4.4 If the hypothetical question is based on case information, the expert should try to give the value of 

507 the evidence given that new proposition. However, there may be propositions (hypothetical questions) 

508 where the expert has no information that informs an opinion about the value of the evidence. The expert 

509 should communicate that he/she is unable to form an opinion about the value of the evidence given that 

510 set of propositions or request additional time to properly consider it.

511 4.4.4.5 The expert should be prepared for multiple hypothetical questions.

512

513

514 Annex A 

515 Recommendations – Supporting Information

516

517 4.1 Sub-sub-source level propositions address “Who is the major or minor contributor?”. An example of 

518 a sub-sub-source proposition pair would be: “Mr. A is the major contributor to the DNA mixture” and 

519 “some unknown person is the major contributor to the DNA mixture”. 

520 4.1.4 The terms “major” and “minor” may translate into issues of activity rather than who contributed the 

521 most/least DNA. Consider a digital penetration case where a major female profile and a minor male 

522 profile is obtained from the male suspect finger swabs. The finding of a major female profile could be 

523 considered strong support if digital penetration occurred rather than casual contact. In this circumstance, 

524 the expert should keep in mind that this information is useful for discriminating activity level propositions 

525 rather than only for discriminating sub-sub-source level propositions. A sub-sub-source level evaluation 

526 leaves the jury the task of evaluating the evidence (i.e., the major profile) without proper guidance unless 

527 the expert also evaluates the evidence using activity level propositions.

528

529 4.1.5.1 It is logically meaningless to suggest that any evidence has value in itself as support for any 

530 particular proposition in isolation. The value of the evidence depends entirely upon the ability to 

531 discriminate between one proposition and another. Therefore, the expert should evaluate the evidence 

532 given at least two different mutually exclusive propositions that are exhaustive in the context of the case.

533 The propositions should be based on task-relevant information such as timelines, alleged activities, and 

534 characteristics of the stain/targeted area. Task-relevant information does not include information such as 

535 prior convictions or allegations, and evidence from other forensic disciplines.

536

537 4.1.5.2 The transposed conditional is any statement by the expert about the probability of the proposition 

538 (disputed fact) given the evidence rather than the probability of the evidence, or findings, given the 

539 proposition. Other terms for this include the prosecutor’s fallacy, source probability error, the ultimate 

540 issue fallacy, and the false positive fallacy. The transposed conditional can occur when considering one 
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541 probability or the ratio of two probabilities. Mathematically, the transposed conditional involves making 

542 statements about Pr(H | E) rather than Pr(E | H). Equally problematic are statements regarding the 

543 probability of the proposition. That is, statements about Pr(H) rather than statements about Pr(E|H).

544

545 Examples of the transposed conditional include: 

546 A. “It is 800 billion times more likely that the DNA originated from Suspect than from an unknown 

547 individual.” 

548 B. “Given these results (suspect’s matching the major DNA profile on the knife) my opinion is that 

549 shaking hands is unlikely.”

550 C. “It is unlikely that shaking hands placed the suspect’s DNA on the knife.”

551 D. “There is only a 1 in 1 million chance that someone else is the source of the DNA profile.”

552 E. “Based on the scientific results, the biological material is probably blood.”

553 F. “Given the results of the presumptive test, it is unlikely that this DNA was deposited one year 

554 ago.”

555

556 An example of an ambiguous statement would be:

557 G. “Shaking hands is unlikely to result in the suspect’s DNA on the knife.” 

558

559 The corrected statements would be:

560 A. The DNA profile is 800 billion times more likely if the DNA originated from Suspect than if it 

561 originated from an unknown individual.

562 B. The DNA profile is less likely if the suspect had shaken hands with the true offender than if the 

563 suspect held the knife.

564 C. The presence of the DNA on the knife (that matches the profile of the suspect) is less likely if the 

565 suspect had shaken hands with the true offender than if the suspect held the knife.

566 D. The probability of the evidence is 1 in 1 million if someone unrelated to the suspect is the source 

567 of the DNA.

568 E. The scientific results are more likely if the biological material is blood than if the biological 

569 material originated from some other body fluid or tissue.

570 F. The results of the presumptive test are unlikely if the DNA was deposited one year ago.

571 G. The presence of the DNA on the knife (that matches the profile of the suspect) is less likely if the 

572 suspect had shaken hands with the true offender than if the suspect held the knife.

573

574 Another correct phrasing is: "The evidence strongly supports the hypothesis that the stain came from 

575 Smith", which is cited in [Evett 1995] and further explained: "The use of the word 'supports' in this 

576 context was proposed by an eminent statistician, H. Jeffreys, and this kind of formulation is, in the 

577 author's opinion, the best available. This is the method which is recommended to scientists within the 

578 Forensic Science Service. Although it successfully conveys the impression that the evidence favors one 

579 hypothesis over the other it is not a probability statement. The strength of the support is based on the 

580 likelihood ratio but the overall probability (or odds) in favor of the hypothesis depends also on the other 

581 evidence.”

582

583 4.1.5.3 There is no standard general probability that applies to all cases alike. Each forensic case has 

584 unique surrounding circumstances (information) and data (analytical results). Therefore, probabilities will 

585 differ from one case/sample to another. This is an expected property of probability.

586

587 4.1.6 Few results in forensic DNA analysis can be communicated in absolute terms. For example, if a 

588 large number of sperm is observed microscopically, this might be considered an unambiguous result as to 

589 the presence of semen. However, there may be some level of uncertainty in forensic DNA casework, such 

590 as a presumptive test for blood, a low-level partial DNA profile (or component of a DNA mixture), or 
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591 questions about activity, transfer, and persistence of DNA. When uncertainty is present, the use of 

592 probabilities is the preferred way to evaluate evidence. 

593

594 4.1.6.1 If sufficient relevant research exists, it is possible to assign numerical values for these 

595 probabilities based on these data and the expert’s knowledge. However, in many situations the scientists 

596 will have to assign a range of notional (personal) probabilities, in the range of 0 to 1. These can also be 

597 expressed in a range of verbal terms such as: ‘very low, low, medium, high, very high’ to convey the 

598 probability. Notional probabilities can be based on expectations derived from data and/or from casework 

599 experience.

600

601 An example of a calculated LR given sub-source propositions is “the DNA profile is 1 million times more 

602 likely if it originated from the person of interest than if it originated from an unrelated individual.”

603 Here, the probability of the numerator (Pr(E|Hp,I)) is calculated separately from the probability of the 

604 denominator (Pr(E|Hd,I)), and the ratio of the two probabilities is presented as the LR. A desirable quality 

605 of this framework is that different models and allele probabilities produce different values, so this 

606 calculation can be specific to a particular case by choosing the most suitable model (e.g., accounting for 

607 subpopulations) and allele probabilities (e.g., representative of the population in the geographical location 

608 of interest) for that case. 

609

610 An example of a notional LR given activity level propositions is “in my opinion, the evidence (i.e., full 

611 single DNA profile matching Mr. A that was obtained from the steering wheel) is more likely if Mr. A 

612 drove the car rather than if he shook hands with the car owner/driver two weeks ago.” (Note that the 

613 choice of calculating a numerical value or presenting a verbal result is acceptable for this example.) At 

614 times, there may be studies (published or internal) available to inform the probability to some degree. 

615 However, each forensic case is unique, and laboratory studies that are repeated under controlled 

616 conditions may have little to do with the circumstances of a given case. In such cases, the probability of 

617 outcomes from studies (typically, frequencies) will need to be modified by the expert to fit the 

618 circumstances of the cases. As mentioned in the Supreme Court decision Daubert v Dow Pharmaceuticals 

619 [509 U.S. 579 (1993)], the court is aware that sometimes there are no studies available for the expert to 

620 use when assigning probabilities:

621 “Some propositions, moreover, are too particular, too new, or of too limited interest to be 

622 published. But submission to the scrutiny of the scientific community is a component of “good 

623 science,” in part because it increases the likelihood that substantive flaws in methodology will be 

624 detected.” (Emphasis added.)

625 Nevertheless, the expert should use probabilities in a robust, transparent, logical, and balanced approach 

626 when assigned probabilities are necessary, which is further alluded to in Daubert:

627 “This entails a preliminary assessment of whether the reasoning or methodology underlying the 

628 testimony is scientifically valid and of whether that reasoning or methodology properly can be 

629 applied to the facts in issue.” (Emphasis added.)”

630 It is for this reason that experts are encouraged to adopt an accepted methodology such as CAI or BNs

631 when evaluating DNA evidence given activity level propositions.

632 4.2.1 If there is no dispute about whose DNA is present, then the LR given sub-source propositions 

633 should no longer be the focus of the expert testimony. Since the parties are now in agreement, it is no 

634 longer relevant. If an LR given sub-source propositions is calculated, then it should be made explicit that 

635 this figure does not relate in any way to questions regarding how or when the DNA was transferred. Not 

636 presenting an LR given sub-source propositions (or RMP) would simplify the task of the jury, since they 
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637 would not have to listen to testimony about an LR or RMP when there is no dispute as to who’s DNA is 

638 present.

639

640 4.2.2.1 See supporting information for 4.1.5.1.

641

642 4.2.2.2 See supporting information for 4.1.5.2.

643

644 4.2.2.2.1 Making a statement about the POI being the source of the DNA is commonly known as 

645 transposing the conditional for an LR, and the prosecutor’s fallacy (which is a form of the transposed 

646 conditional) for an RMP.

647

648 4.2.2.3 See supporting information for 4.1.5.3.

649

650 4.2.2.4 The LR is expressed in a manner that links the numerical value to the DNA profile and not the 

651 person of interest. “This DNA profile is 1 million times more likely if it originated from Suspect rather 

652 than from an unknown unrelated person.” 

653

654 There is an impression that the LR is more difficult to explain to a jury. The use of a verbal scale may be 

655 helpful in conveying the strength of the evidence. 

656

657 A statement similar to “Whatever your belief about whose DNA is present in the sample without my 

658 testimony, based on other information that you have (but I don’t), your belief should be X times greater 

659 with my evidence than it was without my evidence” may be helpful. 

660

661 This statement introduces the concept of prior odds, and communicates that the value of the prior odds 

662 (other evidence) should be determined by the jury. 

663

664 4.2.3 See supporting information for 4.1.6.

665

666 4.2.3.1 See supporting information for 4.1.6.1.

667

668 4.2.3.2 Usually the RMP is a very small number (e.g., 0.000001). However, it is most commonly 

669 expressed as a fraction, e.g., “1 in 1 million”. It may be useful in explaining this to the jury by stating, “In 

670 a general population of 1 million unrelated persons (that we know does not include the offender) the 

671 profile obtained from the evidence is expected to occur one time on average. If multiple populations of 1 

672 million persons are tested, sometimes it may not occur at all, and sometimes more than once, but on 

673 average, only one time.” Care needs to be taken to avoid the defense attorney’s fallacy or the uniqueness 

674 fallacy [Evett and Weir, 1998].

675 4.2.3.3 See supporting information for 4.1.6.2.

676 4.3 For DNA results, source level propositions relate to the attribution of the body fluid or tissue giving 

677 rise to the DNA profile result which matches the person of interest. An example of source level 

678 propositions might be: The bone has originated from Mr. X vs. the bone has originated from an unknown 

679 person unrelated to Mr. X. The results considered are those of the DNA comparison: their value is 

680 calculated given sub-source level propositions. By using source level propositions, the scientist is 

681 elevating the opinion regarding from whom the DNA has originated to from whom a specific body 

682 material has originated [Evett et al., 2002]. 
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683 Note that source level propositions are not the same as the identification of the biological source (i.e., 

684 nature of the body fluid or tissue) of the recovered DNA (e.g., the DNA is from the bone). Both are 

685 important, but the distinction should be maintained by the expert.

686

687 4.3.1 There are times when there is little uncertainty about the nature of the biological material. 

688 Sometimes this is stated by saying “Semen was identified”, such as when thousands of spermatozoa are 

689 observed on a microscope slide. 

690

691 4.3.3 An example of uncertainty about the nature of the biological material is when semen/spermatozoa 

692 testing gives conflicting serological results. For example, this could occur as a negative acid phosphatase 

693 test, a positive immunological test (PSA/p30) and no spermatozoa were visualized microscopically. 

694

695 In the example of only a positive PSA/p30 test, other information may be useful, such as where the 

696 sample came from (i.e., high vaginal sample or underwear) or that the sample ultimately revealed a male 

697 DNA profile in the fraction designed to give DNA results from sperm. In such cases, the use of 

698 probabilities may allow for an opinion on the nature of the biological material to be given in likelihood 

699 ratio form. “This evidence is more likely if the sample contained semen than if it contained some other 

700 form of biological material.” The expert might consider additional information, such as reported injuries, 

701 when giving the overall opinion about the nature of the biological material. The expert should give 

702 serious consideration as to whether or not it is appropriate to give this opinion based on the available case 

703 information. 

704

705 4.3.4 When more than one biological material may be present (i.e., blood and semen, saliva and skin 

706 cells) and a mixed DNA profile was obtained from the sample, the expert should be very careful when the 

707 line of inquiry by the parties is concerned about which donor is the source of which biological material.

708

709 If the source of each biological material is in dispute, or only a small amount of biological material 

710 resulted in a mixed profile, it may be more appropriate to discuss the evidence given activity level 

711 propositions. More value is added at the activity level, and if this activity is not specifically considered, 

712 the jury may make their own evaluation of activity level propositions without proper guidance.

713

714 4.3.5 When the nature of the biological material is in question, making a statement considering only the 

715 DNA profiling results could be mis-interpreted as a comment on the biological material.

716

717 Example: The sample has produced a positive result with Bluestar (a test that reacts with blood). The 

718 DNA profile produced from this trace shows a minor profile matching the suspect and a major profile

719 matching the complainant. The LR statistic is more than 1 billion, meaning that the results are more than 

720 1 billion times more likely if the complainant and suspect are the source of the DNA than if two unknown 

721 persons are. However, if the question of interest is “Whose blood is it?” this large LR for the DNA profile 

722 offers no information as to whose blood it is. The expert should be careful to either make an appropriate 

723 evaluation of the evidence if the blood was from one or both donors, or at the very least a caution that the 

724 LR of 1 billion given sub-source propositions cannot be transferred to the source (blood) level.

725

726 4.3.6 When the biological material is semen, there is usually an inference of sexual activity. If no 

727 evaluation of the evidence is given considering the alleged activities, the jury may be left with no 

728 alternate proposition to consider and assume that sexual activity is the only activity that could have 

729 occurred.

730

731 4.3.7.1 See supporting information for 4.1.5.1.

732

733 4.3.7.2 See supporting information for 4.1.5.2.
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734

735 4.3.7.3 See supporting information for 4.1.5.3.

736

737 4.3.8 See supporting information for 4.1.6.

738

739 4.3.8.1 See supporting information for 4.1.6.1.

740

741 4.3.8.2 See supporting information for 4.1.6.2.

742

743 4.4.2.1 See supporting information for 4.1.5.1.

744

745 4.4.2.1.2 Questions of bias can be minimized when the expert records propositions and expected results 

746 given those propositions prior to testing. This also has an effect on the LR – the probabilities used in 

747 calculating the LR were determined independent of the data, and the results end up supporting one 

748 proposition compared to the other with little to no post results influence of the data. It should be noted 

749 that the propositions are not immutable, and that they may require refinement or alteration during the 

750 course of the case. 

751

752 A simple statement such as “Per case information, one proposition will be that Suspect touched the item 

753 directly, and the alternative proposition will be that Suspect was interacting with a group of people, and 

754 someone else who had contact with Suspect touched the item” may be sufficient when there is 

755 information that Suspect was with a group of people that had some exposure to the evidence item. 

756

757 4.4.2.1.2.1 An example of expected results may be “I would expect to see DNA only from Suspect if the 

758 first proposition is true; I would generally expect to recover DNA from Suspect and additional 

759 contributors if the alternative proposition is true.”

760

761 The expert should be cautious of assigning any probability of 1 or 0 (zero) as that indicates no uncertainty 

762 and leaves no room for the other proposition to have any part in the evaluation of the evidence. These 

763 situations are rare in practice. (See the discussion on “possible” at Annex A 4.4.4.2 in this document.) 

764

765 4.4.2.1.2.2 This process follows a recognized model for casework assessment and interpretation [ENFSI, 

766 Cook et al. (1998) CAI paper]. A number of steps are conducted: 

767 1. The scientist will consider the expected outcomes of the test: for example, no DNA profile 

768 matching the person of interest, or DNA matching a person of interest.

769 2. (Notional) probabilities are then assigned to the expected outcomes given the truth of the 

770 proposition. This is repeated for each of the propositions in the case.

771 3. The relative weight of evidence (LR) that the findings provide is then derived through division of 

772 the probabilities.

773

774 The examples presented here are very simple and minimalistic. It is recommended that more formal 

775 approaches be adopted such as CAI using a table of probabilities or Bayesian network software. These 

776 approaches will require training and effort, but the tables or graphs that result are easily used to 

777 communicate the thought process to the decision maker. They also allow for transparency and ensure that 

778 conclusions are robust (i.e., sustain scrutiny by peers and cross-examination by counsel during the trial).

779

780 The use of CAI and BNs are strongly recommended, as these methods are recognized in published studies 

781 for the evaluation of DNA evidence given activity level propositions. Several are listed in Annex C as a 

782 starting point. Because it is expected that different experts may present different likelihood ratios for 

783 DNA evidence given activity level propositions (as is the case for likelihood ratios given sub-source level 
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784 propositions when using different models), the courts have recognized that using a rigorous framework is 

785 imperative for the evaluation of the testimony of the expert by the court. In Kumho Tire Co. v Carmichael 

786 [526 U.S. 137 (1999)], the Supreme Court has emphasized that the method applied by the expert must

787 stand up to scrutiny, whether from published studies and data or notional probabilities based on personal 

788 observations and experience gained through evidence testing:

789

790 “It is to make certain that an expert, whether basing testimony upon professional studies or 

791 personal experience, employs in the courtroom the same level of intellectual rigor that 

792 characterizes the practice of an expert in the relevant field.” (Kumho Tire Co. v Carmichael, 526 

793 U.S. 137 (1999))

794

795 4.4.2.1.3 “Transfer” is not a part of the actual actions alleged in the proposition. Anytime DNA moves 

796 from point A to point B, transfer has occurred, but it is not descriptive of the actions that may have been 

797 taken by the parties. The mode of transfer is implicit in the activities considered in the propositions.

798

799 Propositions are addressed by the court. If the term “transfer” is woven into the propositions, then it 

800 would mean that the court has to assess transfer. However, as forensic scientists have knowledge on that 

801 topic, it should be part of the assessment, not of the propositions.

802

803 An incorrect example of using “transfer” in the propositions is:

804

805 “The presence of the DNA on the knife is more likely if direct transfer occurred than if indirect 

806 transfer occurred.”

807

808 This is incorrect because the term “transfer” is a part of the propositions. “Transfer” is not a proposition; 

809 it is a mechanism.

810

811 Another incorrect example is:

812

813 “It is more likely that direct transfer occurred than indirect transfer.”

814

815 This statement is incorrect for two reasons: 1) it transposes the conditional, and 2) the term “transfer” is a 

816 part of the propositions.

817

818 The correct statement would be:

819 “The presence of the DNA on the knife is more likely if the suspect held the knife rather than if 

820 the suspect had shaken hands with the true offender.”

821

822 It would be incorrect to say, “This evidence is more likely if primary transfer occurred than if secondary 

823 transfer occurred.” It would be correct to say “This evidence is more likely if Mr. A handled the gun than 

if Mr. A didn’t handle3 824 the gun.” While there is a ‘primary’ transfer for the first proposition, and a 

825 ‘secondary’ transfer in the alternative proposition, the actual propositions are based on the allegations of 

826 the parties and communicate more to the finder of fact.

827

828 4.4.2.1.4 Typically, the expert is aware of the prosecution’s version of events at the time of evidence 

829 testing. The proposition is formulated for the purpose of assigning the probability of the evidence if this 

830 proposition were true.

3

In the case information, one will describe what is meant by ‘not’ (e.g, The gun was touched by the 

police with the same gloves that were worn to arrest Mr. A). 
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831

832 There may be times where the prosecution proposition is not supported by the evidence and the evidence 

833 is more probable given the defense proposition. For example, when the sexual activity in question 

834 occurred 72 hours ago according to the prosecution, yet many sperm heads are seen from a vaginal swab. 

835 This does not mean the proposition is or is not true. Rather, the defense may have an alternative time 

836 frame (e.g., 24 hours rather than 72 hours) for the sexual activity that is better supported by the evidence. 

837 If this is the situation, then the expert should explain that the evidence is more likely if the sexual activity 

838 occurred one day prior to collection than if it occurred three days before collection.

839

840 4.4.2.1.5 The alternative proposition must be different and mutually exclusive from the prosecution’s 

841 proposition or version of events. That is, if a proposition is “true,” then the alternative proposition cannot 

842 be “true” and vice versa. The alternative proposition must either benefit the defense, as understood by the 

843 expert based on the available information, or if provided, reflect the position of the defense.

844

845 4.4.2.1.5.1 It is crucial to be aware that in the United States (as well as in other countries) there is no 

846 requirement for the defense to make a statement of any sort. This does not relieve the expert from the 

847 responsibility of evaluating the evidence using a proper Bayesian framework with competing 

848 propositions. 

849

850 4.4.2.1.5.2 When there is no specific information, the expert has no choice but to adopt a proposition for 

851 the defense. This is necessary to give a balanced evaluation of the evidence. 

852

853 Examples of proxy propositions for the defense may sometimes be found in surrounding activities that are 

854 not in dispute. Examples in a sexual assault case may include having dinner together, social contact, 

855 having previous intercourse, holding hands prior to touching an item, and so on. On occasion, the expert 

856 may have little choice but to choose a proxy of “not sex” or “not touching the knife” if there is little to no 

857 information available. Be aware that these “not X” proxy propositions can be problematic both for the 

858 expert (difficult to assign a probability of the findings given this proposition) and the defense (may lead to 

859 relatively large LRs which favor the prosecution proposition.) 

860

861 4.4.2.2 See supporting information for 4.1.5.2.

862

863 4.4.2.3 See supporting information for 4.1.5.3.

864

865 4.4.2.3.1 There is always a concern about bias when the expert needs case information to evaluate the 

866 evidence. Only task-relevant information should be considered (e.g., relationships of the individuals, the 

867 alleged activities, the time between the alleged events and evidence collection). Information such as prior 

868 convictions, whether the suspect was recognized or not, and the presence of other evidence are examples 

869 of information that is not useful and potentially harmful. Some laboratories will filter the information

870 given to the analysts, so that the person(s) assessing the findings will only be given task relevant 

871 information. 

872

873 The so-called “black box” expert where the expert testifies only to analytical results with no conditioning 

874 information has its own dangers and could be misleading to the decision maker. In the following 

875 examples, the expert needs information about the circumstances of the case and the propositions of the 

876 parties in order to give any value to the evidence that was examined. 

877

878 One example would be where the expert testifies only to “the major semen DNA profile” and doesn’t 

879 evaluate the evidence given activity level propositions. If the question of interest to the court is the 

880 timeline of activity (e.g., 12 hours or 72 hours) and no comment is made beyond “major semen DNA 

881 profile” the jury does not have the information necessary to evaluate the real issue before the court. 
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882

883 Another example might be “Complainant is included as a DNA contributor on the item.” If there is no 

884 dispute that the DNA profile came from the Complainant, because the question of interest is whether 

885 direct or indirect contact occurred between the complainant and the object, this statement by the expert 

886 adds no value to the case. 

887

888 4.4.2.4 The principles used to assign a likelihood ratio provide a recognized framework to communicate 

889 the value of the evidence given activity level propositions [Gill et al., 2020, ENFSI Guidelines 2015]. It is 

890 common to consider propositions at the activity level in forensic DNA testimony. Activity levels can be 

891 implied at the sub-sub-source level (e.g. complainant corresponds to major profile on Suspect fingernails 

892 in a digital penetration case) or the source level (e.g. Suspect matches the semen profile). It can also be 

893 direct testimony given activities of interest from one or both of the parties in the case. There is a high 

894 level of uncertainty in most cases involving activity. The likelihood ratio using probabilities of the 

895 evidence given competing propositions is the preferred way of evaluating evidence. It is important to use 

896 the Bayesian framework where the expert focuses on the likelihood ratio and lets the decision maker 

897 (jury/judge) consider the other evidence (prior odds). The decision maker – and not the expert – is 

898 responsible for the final evaluation of the propositions (posterior odds). Oftentimes the judge instructs the 

899 jury on this issue, even if these exact terms are not used. 

900 4.4.3 See supporting information for 4.1.6.

901

902 4.4.3.1 See supporting information for 4.1.6.1.

903

904 4.4.3.2 See supporting information for 4.1.6.2.

905

906 4.4.4 Oftentimes, whether or not there is any statement made prior to court regarding the activities, 

907 counsel will ask the expert a hypothetical question about DNA transfer and persistence (e.g., “Is it 

908 possible for one person to handle an item, and then a second person handle that same item and transfer the 

909 DNA of person 1 to a second item?”). 

910 ‘Whose DNA is it?’ is an entirely different question from addressing ‘How and when did the DNA get 

911 there?’ This means that any sub-source statistics quoted have no bearing on the issues regarding how or 

912 when the DNA was transferred. In a scientific context, it might not be known for certain ‘how’ or ‘when’ 

913 DNA was transferred to an object.

914 The ‘how’ and ‘when’ questions involve issues affecting the transfer of DNA from one place to another 

915 and a consideration of how long the DNA might have persisted on the item once transfer has taken place. 

916 In addition, the attribution of the DNA to a particular body fluid will also form an important part of these 

917 considerations as well as background levels of DNA and possible contamination.

918 Any DNA transfer event requires four factors to be considered and fulfilled:

919 i) there needs to be a source of DNA.

920 ii) there needs to be a mechanism by which the DNA is transferred.

921 iii) there needs to be an opportunity for the DNA to be transferred by the proposed mechanism. 

922 iv) the DNA must be in sufficient quantity and quality to persist and be recovered from the item.

923 Consideration of these points needs to be undertaken within the context of the case information.
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924 4.4.4.1 Most often, a hypothetical question is asked at court in an attempt to explain the evidence after the 

925 evidence (results) are known. The expert should understand that an explanation is not a proposition. 

926 However, a hypothetical question on activity/transfer/persistence issues by the defense is an engagement 

927 about the activity and should be considered in the same manner as a more formal proposition. When this 

928 happens, the best answer the expert can give is to evaluate the evidence if the prosecution’s proposition 

929 occurred compared to if the hypothetical situation just asked about occurred. (See the discussion on 

930 “possible” at Annex A 4.4.4.2 in this document.)

931

932 The expert needs to be aware that such “after the fact” hypothetical questions (explanations) will often 

933 lead to an LR of 1. That is to say, after the results are obtained, there may be multiple explanations that 

934 can be put forward that could yield results similar to the observed results. For example, if the prosecution 

935 proposition involved non-consensual sex and the defense proposition involves consensual sex, the 

936 probabilities of the result given either of those propositions would be equal. This assessment of the DNA 

937 evidence offers no value to the proceedings.

938

939 4.4.4.2 The forensic scientist should not deal with “possibilities” as this is a mathematically meaningless 

940 term. In the context of a hypothetical case, “possible” typically means any non-zero probability. However, 

941 probabilities are discrete values between 0 and 1, and if there are several outcomes to consider, the sum of 

942 all probabilities must equal 1. The opposite term for “possible” is usually “not possible” or “impossible”. 

943 In mathematical terms, this can be expressed as (1 – possible = impossible) where impossible has a 

944 probability of zero. Therefore “possible” cannot be dealt with mathematically by the scientist when 

945 evaluating evidence. 

946

947 When a question is asked if something is “possible” one or more of the following may be useful:

948

949 “As a scientist, I can only deal with probabilities as all things may be possible.”

950

951 “Possible is any non-zero probability, yet what is relevant when evaluating evidence is to 

952 consider the relative probability assigned to that evidence given each of two different 

953 propositions.”

954

955 “I can only answer your question if I consider relative probabilities of the evidence considering 

956 two options, not by stating some unknown intrinsic value of a single proposition in isolation.”

957

958 “The only way to put “possible” into context is to compare the relative probabilities of the 

959 evidence given each of two different propositions. We can never be certain about the probability 

960 of the evidence for a given event, but we can provide context by comparing the probabilities 

961 given each of two different and mutually exclusive propositions.”

962

963 "There may be a lot of ways to explain how DNA was deposited somewhere. I can't properly 

964 assess every possibility on its own. My job is to compare the probabilities of getting this type of 

965 DNA profile under two different scenarios that are useful and reasonable to the case. The 

966 outcome may change when different scenarios are compared, but there is not much value in 

967 considering only one scenario by itself."

968

969 4.4.4.3 This may be the biggest challenge when considering activity level propositions. Often 

970 hypothetical questions are directly asked in reference to the activity, or the expert is asked to provide 

971 activities that could result in the type of testing results found in the case. The expert needs to recognize 

972 these questions as the transposed conditional (See 4.1.5.2) and attempt to answer appropriately by 

973 referring to the evidentiary findings if that activity has occurred. Answering these questions directly is not 

974 the role of the expert. We list some examples and possible responses below:
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975

976 Q1 “What are some other explanations for this data?” – Requires the expert to know the other 

977 evidence in the case in order for the explanation to have a prior. This is outside the scope of the 

978 expert.

979

980 “You are asking me to provide explanations after I know the data. This is outside my role. If you 

981 have additional explanations that interest you, I can give you an opinion on whether or not the 

982 evidence supports what you are interested in.”

983

984 “I’m not aware of any case information that would allow me to make any speculations like that.”

985

986 Q2 “Is it possible that the DNA got there by secondary transfer?” – Any simple answer 

987 (yes/no/maybe) is a comment on the probability of the proposition, not the probability of the 

988 evidence if the proposition were true. Therefore any “simple” answer is a transposed conditional. 

989 Also, see the problems with “possible” addressed in 4.4.4.2.

990

991 “I’m certain that if we were able to run some trials, we could determine how often and under 

992 what circumstances DNA is detected from a secondary transfer event. However, what is relevant 

993 is whether the observed DNA profile is more likely if an object was handled by the person of 

994 interest or if he did not handle it but had contact with an unknown person who did.”

995

996 “When you ask if that’s possible, the answer is yes. However, if we do not attach any value to the 

997 results then this is of little assistance. Is it possible to win the lottery 6 times, yes it is possible. Is 

998 it possible that if I flip a coin it will land on heads, yes it is possible. But as you see the 

999 probabilities are very much different. Here, in this specific case, in order to be of assistance to the 

1000 court, I would need to give the value of the DNA results considering the two competing views of 

1001 the parties.”

1002

1003 4.4.4.4 When answering a hypothetical question, the expert should be clear as to whether or not he/she 

1004 has relevant case information related to the question. While the expert should never give an opinion about 

1005 the other evidence (which represents prior odds in the Bayesian approach), it is prudent to consider the 

1006 case information that has been presented to the expert. The expert needs to be aware that giving an 

1007 opinion on the weight of evidence for a given pair of propositions may imply some activity has occurred 

1008 when there is no other evidence supporting that particular proposition.

1009

1010 The use of a likelihood ratio does not generally imply that one of the two propositions considered must be 

1011 true. Though the considered propositions are those deemed most relevant, they do not need to be 

1012 exhaustive, so both propositions could be false. The likelihood ratio says nothing about propositions other 

1013 than the two that were considered.

1014

1015 If the hypothetical seems “reasonable” such as ‘dancing together’, the expert may not have specific case 

1016 information about dancing but is aware that V and S were on a date. In this case, the expert may feel that 

1017 he/she has an opinion that could add value.

1018

1019 “I’m unaware of any specific information about dancing, but I’m aware that V and S were 

1020 together on a date for the evening. Therefore, I am comfortable giving an opinion that the 

1021 evidence is more/less likely if they danced together than if X were true,” where X is some other 

1022 proposition.

1023

1024 If the hypothetical seems “unreasonable” such as multiple steps of transfer that require specific conditions 

1025 for which no information is available to the expert to consider, the expert may or may not be able to 
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1026 answer directly. Consider an apparent outlandish scenario of semen on a vaginal swab with a hypothetical 

1027 question of V using contaminated toilet paper because the holder was empty [Gittelson 2016]:

1028

1029 “I am unaware of any information in the case where it was claimed the toilet paper roll was empty 

1030 so V used discarded tissue from the bathroom trash. However, in my opinion, recovering a DNA 

1031 profile from sperm cells is more likely if sexual activity occurred than if contaminated tissue was 

1032 the source of the sperm cells.”

1033

1034 “I have not been presented with this specific alternative in this case. I would need some more 

1035 information and some time to prepare an answer if this is relevant.”

1036

1037 The expert should remember that any transfer mechanism of DNA could have a non-zero probability 

1038 given the right circumstances. Because the specific case information matters, the expert should discuss the 

1039 limits of any opinion with the decision maker and be clear that hypothetical questions should fit within 

1040 the overall case information.

1041

1042 4.4.4.5 Often an expert is asked a series of hypothetical questions. The expert should attempt to make the 

1043 court understand that the best way to answer these questions is to consider the probability of the results 

1044 obtained given each of two different options. When one hypothetical question involves a significant 

1045 question (such as “touch DNA” moving to a body fluid), the expert may need to ask for a moment to 

1046 consider the answer. Finally, if the answer were to involve much effort, perhaps it is best to ask for a 

1047 recess. It is also possible that the expert should answer by saying he/she has no ability to offer an opinion 

1048 on the probability of the evidence given that scenario.

1049

1050 Annex B

1051 (informative)

1052 Examples

1053

1054 The following examples have been included in an attempt to give guidance on how one might testify at 

1055 trial when the court has interest in the evaluation of the evidence given activity level propositions. In the 

1056 examples presented, both parties have described activities that could have legitimately led to the transfer 

1057 of DNA. These examples have been simplified and may exclude issues such as lab-derived 

1058 contamination, background DNA, or underlying issues with the testing methodology. The reader must be 

1059 aware that in real life cases these issues might need to be considered. 

1060

1061 It is beyond the scope of this best practices recommendation to give detailed instructions and strategies 

1062 for specific cases examples. For more information, consult the references cited throughout this document.

1063

1064 It is recommended that the expert include the following information as general background before 

1065 addressing his or her opinion about the evidence given propositions at the activity level of the Hierarchy 

1066 of Propositions:

1067

1068 ● “In order to assist the court, I have followed a recognized framework for evidence evaluation by 

1069 considering the probability of observing the results in this case given two alternative propositions. 

1070 These reflect the prosecution and defense standpoints as understood by me.” 

1071

1072 ● (This statement may be needed as well:) “I have no specific information that would allow me to 

1073 infer what alternative activities would have taken place. To assess the biological results, I have 

1074 adopted a proposition that incorporated case information that is not disputed between the parties, 

1075 to the best of my knowledge.”
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1076

1077 ● “The approach to the examination and interpretation of the findings in a case is crucially 

1078 dependent on the information made available to me and the propositions considered. If any of the 

1079 information I used in the evaluation were to change, is incorrect, or incomplete, then I will 

1080 reconsider the evaluation of the evidence and this may result in a different overall opinion.”

1081

1082 The examples are intended to model the thought processes and the proper presentation of the opinion 

1083 using a Bayesian framework. The process used in the examples follow the recognized framework for 

1084 casework assessment and interpretation (CAI) [Cook et al., 1998 CAI] as described in the introduction of 

1085 this document.

1086

1087 Both qualitative and quantitative approaches are acceptable. It is recommended that if a purely qualitative 

1088 approach is used consisting of verbal qualifiers (such as more likely/very much more likely, high/low), 

1089 the expert should give context by at least discussing the word choices, and some indication as to the 

1090 relative scale of each. Because words may mean different things to different people, it is recommended 

1091 that a quantitative evaluation be used whenever possible. However, there may be situations where this is 

1092 not possible as there is no available research that is directly related to the case activities of interest. In 

1093 addition, there may be jurisdictional preferences between qualitative and quantitative evaluations that 

1094 must be considered. Much more information may be gained by consulting the references cited in this best 

1095 practices recommendation.

1096

1097 The examples are intended to move through a progression from entirely qualitative in the first example to 

1098 a fully quantitative example at the end. The examples between the first and last are intended to show a 

1099 progression that incorporates published literature, and some form of numerical quantification of the 

1100 strength of the evidence – even if that is merely an acknowledgement that a numerical evaluation cannot 

1101 be calculated. While a qualitative evaluation is acceptable, and sometimes the only available option, it is 

1102 recommended that all efforts be made to offer a quantitative assessment whenever possible.

1103

1104 ● Example 1 is purely qualitative and makes no mention of research.

1105 ● Example 2 is qualitative, although there is mention that a numerical evaluation cannot be 

1106 performed. Some relevant research is mentioned, but it is unbalanced and only related to one 

1107 proposition. Therefore, a numerical evaluation is not presented.

1108 ● Example 3 is a continuation of Example 2 and serves to show how new/additional information 

1109 may have a significant effect on the expert’s opinion.

1110 ● Example 4 presents a quantitative number, but it is based on a qualitative scale. This is one way 

1111 to bridge the gap between qualitative and quantitative evidence evaluation, and further serves to 

1112 enforce the criteria of transparency, balance, logic, and robustness.

1113 ● Example 5 is an attempt to show a quantitative approach using relevant literature. However, the 

1114 numerical LR presented in #5 has been simplified for clarity and an example of the craft:

1115 o Simplified propositions were considered.

1116 o No attempt was made to find current literature that may exist, the citations are fictitious.

1117 o A Bayesian network was not used.

1118 o Sensitivity analysis was not done. 

1119

1120 Case scenario 1.

1121 Information: 

1122 Ms. Y alleges that she awoke to find Mr. X masturbating over her, and that he ejaculated on her clothing. 

1123 Mr. X denies the allegation, but stated he argued with Ms. Y, and did at one point spit at her. A night 
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1124 shirt with a stain has been provided as evidence and was collected within a few hours of the event. Mr. X 

1125 is not vasectomized.

1126

1127 Question: Did Mr. X ejaculate onto Ms. Y’s clothing?

1128 Propositions considered:

1129 ● Mr. X ejaculated on Ms. Y.

1130 ● Mr. X spat on Ms. Y.

1131

1132 Laboratory testing:

1133 a) Visible stained area on shirt, this area also shows fluorescence using a forensic light source.

1134 b) Positive result for the presence of amylase. 

1135 c) P30 test returned a negative result. 

1136 d) No sperm were found via microscopic analysis of the stain. 

1137 e) After quantification, the DNA extract required a 10-fold dilution before amplification. 

1138 f) A DNA profile was recovered from the stain on her nightshirt.

1139 g) The DNA typing results showed a major profile matching Mr. X, and Ms. Y accounts for the minor. 

1140

1141 Assumptions: 

1142 Any DNA matching Mr. X on the nightshirt has originated from him. 

1143

1144 Oral Testimony: 

1145 I have evaluated the evidence with respect to two propositions: either Mr. X ejaculated on Ms. Y, or Mr. 

1146 X spat on Ms. Y. If ejaculation had occurred, then I would expect the P30 test to be positive, and perhaps 

1147 there to be sperm cells present. If spitting had occurred, then I would expect the amylase test to be 

1148 positive. The second proposition would give rise to the results obtained in this case, whereas the first 

1149 would not. Therefore, I consider that the results would be very much more likely to be observed if Mr. X 

1150 spat on Ms. Y rather than if Mr. X ejaculated on Ms. Y.

1151

1152 Evaluation:

1153 a) Based on case information, the analysis was limited to semen and saliva testing. Laboratory validation 

1154 shows that when semen is present (with similar timing as in this case), one expects to have a P30 positive 

1155 test and to observe sperm heads using microscopy. (Case information exists that Mr. X has not been 

1156 vasectomized). When saliva is present (again within the same timing), we expect a positive amylase test. 

1157 However, amylase testing is not a specific or confirmatory test. I do not expect DNA recovered from an 

1158 item of evidence to require a 10-fold dilution prior to amplification as a result of normal social contact (or 

1159 background level DNA). Generally, only body fluids and tissue samples require dilution. 

1160

1161 b) Considering the first proposition. I do not expect to observe negative semen testing results if 

1162 ejaculation occurred. Based on a timely collection of the evidence and proper handling to preserve 

1163 biological material, I would expect positive semen testing results if semen were present on the shirt. I do 

1164 not expect such quantities of DNA as recovered here to be present as background. The probability of 

1165 these results if Mr. X ejaculated on the shirt are considered to be extremely low.

1166

1167 c) Considering the alternative proposition. The results in this case (large quantity of DNA; saliva positive 

1168 test, negative semen test) are what I would expect if spitting took place. I do not expect such quantities of 

1169 DNA as recovered here to be present as background. The probability of these results if Mr. X spat on the 

1170 shirt are considered to be extremely high.

1171

1172 d) In summary, only one of these activities would be expected to give rise to the findings in this case. 

1173 Therefore, in my opinion, the results are very much more probable if spitting took place rather than if 

1174 ejaculation took place.
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1175

1176 Case scenario 2:

1177 Information: 

1178 Mr. X and Ms. Y were socializing in a public bar. They were part of a group who drank and danced 

1179 together. Ms. Y has accused Mr. X of forcibly penetrating her with his fingers while dancing. Mr. X has 

1180 not washed his hands, and he has invoked his right to remain silent.

1181

1182 Question: Did Mr. X digitally penetrate Ms. Y?

1183 Propositions considered:

1184 ● Mr. X digitally penetrated Ms. Y and also socialized (danced) with Ms. Y.

1185 ● Mr. X simply socialized (danced) with Ms. Y.

1186

1187 Laboratory testing: 

1188 a) Right- and left-hand fingers/fingernails of Mr. X swabbed ~5 hours after the alleged incident.

1189 b) DNA Right hand fingers/fingernails of Mr. X give a mixed DNA profile. The major component profile 

1190 matches Ms. Y; the minor component profile matches Mr. X.

1191 c) The left-hand fingers/fingernails gave a DNA profile matching Mr. X only.

1192 d) No laboratory testing has been performed to establish the body fluid or cell type.

1193

1194 Assumptions: 

1195 The source of the DNA is not disputed (i.e., the DNA is from Mr. X and Ms. Y). 

1196

1197 Oral Testimony: 

1198 I have evaluated the evidence in this case with respect to two propositions: either Mr. X digitally 

1199 penetrated Ms. Y during an evening spent socializing/dancing, or Mr. X simply socialized/danced with 

1200 Ms. Y. My opinion, considering the probability of observing the results under the different propositions, 

1201 is that the results would be considered much more likely to be observed if the first proposition were true 

1202 rather than the second. I am unable to provide a quantitative measure of the relative strength of support 

1203 that the findings provide. 

1204

1205 Evaluation: 

1206 a) Considering the first proposition. The vaginal environment contains an abundance of DNA rich cells. 

1207 Insertion of a finger would require some force and, in conjunction with the wet nature of the cells, would 

1208 promote DNA transfer to a finger. This would likely result in a good quality DNA profile matching Ms. 

1209 Y, as were the findings in this case. The absence of a similar result on the other hand is also considered 

1210 significant under this proposition. This is because there is an expectation of similar results on both hands 

1211 if Mr. X and Ms. Y only had typical social contact. The expectation of these results if digital penetration 

1212 occurred is so high that for simplicity's sake, one can ignore the “social contact” portion of this first 

1213 proposition. If the first proposition is true, then the probability of obtaining the results in this case would 

1214 be considered ‘high’. 

1215

1216 b) Considering the alternative proposition. Social situations will provide opportunities for direct and 

1217 indirect DNA transfer through successive contacts with common objects and individuals. In this situation, 

1218 it would be my expectation that these activities would generate complex mixed DNA profile results from 

1219 many contributors, since Mr. X and Ms. Y were part of a larger social group. Additionally, these results 

1220 would be expected on both hands. The probability of obtaining the results found in this case (all DNA 

1221 accounted for by Mr. X and Ms. Y, and only on one hand) if this proposition were true would therefore be 

1222 considered ‘low,’ and against expectations.

1223

1224 c) Additionally, research indicates that the chances of finding a high-quality foreign DNA profile on a 

1225 person’s fingernails, other than from someone with whom they have an intimate relationship, is low.
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1226

1227 d) In summary, both activities would give rise to the findings in this case, but with different expected 

1228 probabilities. When considering the probability of observing the results under the different propositions, 

1229 the results would be considered much more likely to be observed if the first proposition were true rather 

1230 than the second. 

1231

1232 Case Scenario 3:

1233 This is a continuation of Case Scenario 2. However, after the initial expert testimony by the DNA expert, 

1234 defense later presented a witness that testified during the evening he saw Ms.Y lick the index finger of 

1235 Mr. X’s right hand after he dipped it into her drink. 

1236

1237 Note: The expert was not incorrect in Case Scenario 2, but rather the information provided was 

1238 incomplete with respect to the alternative proposition. In light of the new information introduced by the 

1239 defense, it is imperative (and logical) that the expert re-assess the results given the new information. 

1240 Although Mr. X has made no statement, the defense is entitled to introduce evidence as fits their case 

1241 strategy, and this may—or may not—affect the evaluation of the evidence by the DNA expert.

1242

1243 Question: Did Mr. X digitally penetrate Ms. Y?

1244 Propositions considered:

1245 ● Mr. X digitally penetrated Ms. Y. 

1246 ● Ms. Y licked the index finger of Mr. X and no digital penetration took place. 

1247

1248 Laboratory testing:

1249 See Case Scenario 2; there are no additional testing results.

1250

1251 Assumptions: 

1252 All previous assumptions apply.

1253

1254 Oral Testimony: 

1255 I have evaluated the DNA results in this case with respect to two propositions: either Mr. X digitally 

1256 penetrated Ms. Y, or Ms. Y licked the index finger of Mr. X and no digital penetration occurred. The fact 

1257 that Ms. Y and Mr. X danced together and spent the evening together in a social setting is taken into 

1258 consideration under both propositions, as this is not contested. The probability of observing the DNA 

1259 results under the different propositions would be generally considered the same when considering the 

1260 nature of the body fluids and cell types (vaginal cells/fluids versus buccal cells or saliva). I would expect 

1261 the same findings for either proposition. Therefore, the results are uninformative or neutral and do not 

1262 help address the issue.

1263

1264 Evaluation:

1265 a) Considering the first proposition. The probability of obtaining the result remains ‘high’ as stated in 

1266 Scenario 2 for the proposition that Mr. X digitally penetrated Ms. Y’s vagina.

1267

1268 b) Considering the alternative proposition. The new information changes the probability of the results 

1269 under the alternative proposition. Insertion of a finger into the oral cavity, as with the vagina, is expected 

1270 to provide a good opportunity for the transfer of cells/DNA. The probability of obtaining the results under 

1271 this proposition, with conditions of retention and persistence being the same as that of the first 

1272 proposition, is considered ‘high’ and equal to the probability of the results if there were digital 

1273 penetration.

1274

1275 c) In summary, I would expect the same findings under both propositions. Notwithstanding that the nature 

1276 of the body fluid cannot be determined, the probability of observing the DNA results under the different 
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1277 propositions would be generally considered the same. Therefore, the results are uninformative or neutral 

1278 and do not help address the issue.

1279

1280 Case scenario 4

1281 Information:

1282 Motor vehicle collision – airbag deployed. One person (driver) is seen leaving the scene. 

1283 Mr. X is charged with the offence. He states that:

1284 ● he owns the car, and therefore, his DNA will be on surfaces within the car

1285 ● a cup was present in the central console of the car which he has drunk from

1286 ● someone stole the car prior to the collision.

1287

1288 Question: Who was driving the car at the time of the collision?

1289 Propositions considered:

1290 ● Mr. X was driving the car at the time of the collision.

1291 ● An unknown person was driving the car at the time of the collision, not Mr. X. 

1292

1293 Laboratory testing: 

1294 a) Airbag tested for saliva and DNA.

1295 b) Central front region contained an area of staining that yielded a strong amylase positive result. 

1296 c) The area of assumed saliva gives a good quality single source profile matching Mr. X. 

1297 d) The LR is 1 billion. That is, it is 1 billion times more likely to observe the DNA evidence if it 

1298 originated from Mr. X rather than if it originated from someone else unrelated to Mr. X.

1299

1300 Assumptions:

1301 a) The determination of saliva is not under dispute.

1302 b) The DNA has originated from the saliva tested.

1303 c) Saliva has originated from Mr. X.

1304

1305 Oral testimony: 

1306 I must first point out that the stated likelihood ratio for the DNA profile (i.e., 1 billion) cannot be applied 

1307 to the evaluation of the evidence given the propositions about who was driving the car. I have considered 

1308 this evidence with respect to two possible propositions: Mr. X was driving the car at the time of the 

1309 collision, or an unknown person was driving the car at the time of the collision. It is my understanding 

1310 that Mr. X is the normal driver. It is my opinion that both scenarios could give rise to the findings in this 

1311 case. However, when considering the probability of observing the results under the different propositions, 

1312 the results would be considered much more likely to be observed if the first proposition were true rather 

1313 than the second. This is because I have a high expectation of these results given one proposition, and a 

1314 low expectation given the other. 

1315

1316 I can illustrate my opinion by assigning a numerical value, for example, of 0.9, or 90% to the term ‘high’ 

1317 and 0.01, or 1%, to the term ‘low.’ So another way to express my opinion is to say that the evidence 

1318 recovered from the airbag is in the order of 90 times more likely if Mr. X were driving the car at the time 

1319 of the collision than if someone else were driving, and Mr. X’s DNA was present due to car ownership. I 

1320 have simplified this mathematical example by not considering that the cup Mr. X had in the car was an 

1321 alternative source of the DNA on the airbag. In my opinion, this possibility is so remote as to have no 

1322 influence on my calculation.

1323

1324 Evaluation information:

1325 a) The two probabilities assigned for the observed results given each proposition allow the calculation of 

1326 a likelihood ratio. This likelihood ratio represents the relative strength of support that the findings provide 

1327 given the stated propositions. I am giving a qualitative opinion, as I am unaware of any published studies 
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1328 related to DNA profiles recovered from airbags and car owners. However, I am aware of numerous 

1329 studies involving the comparison of DNA transferred by saliva and other body fluids compared to skin 

1330 cells, and also of studies involving general background levels of DNA. 

1331

1332 b) Considering the first proposition. If Mr. X were driving, then it is reasonable to consider that the 

1333 driver’s airbag has deployed into his face with some force. There is an expectation that this would have 

1334 resulted in the transfer of his saliva, and therefore his DNA, to the central part as a single source DNA 

1335 profile as were the findings in this case. The probability of the evidence given this proposition is 

1336 considered very high (i.e., close to 1), because these results are expected if this proposition is true. I have 

1337 assigned a probability value of 0.9 to this proposition as an illustration of ‘high.’

1338

1339 c) Considering the alternative proposition. If some other person were seated in the driver’s seat, then:

1340 ● saliva is present in the area tested as a result of the airbag being deployed into the unknown 

1341 driver’s face and

1342 ● the DNA profile of the ‘true’ driver (i.e., the driver at the time of the collision) could, by some 

1343 very remote possibility, match that of Mr. X. This is not expected to be the case; therefore, I have 

1344 to accommodate for the profile of the ‘true’ driver not being detected.

1345 ● Since the airbag is entirely enclosed within the steering column before deployment, there would 

1346 be no reasonable opportunity for DNA from Mr. X to be transferred to it as a result of him 

1347 sneezing over it or being in his car on a previous occasion. Therefore, we may consider the 

1348 DNA/saliva was transferred through contact with areas of the car where his DNA is expected to 

1349 be present when the airbag was inflating or deflating. Additionally, in my opinion, it would be 

1350 difficult to conclude that the saliva detected on the airbag could be explained from transfer of 

1351 saliva from the rim of a cup in the cup holder. This saliva would have to be wet to facilitate 

1352 transfer and would have to be present over a similarly shaped and equivalent area as detected on 

1353 the airbag (i.e., not commensurate with the rim of a cup).

1354 ● In this situation the chances of observing the DNA profiling results would be considered ‘low.’ 

1355 This is because the second alternative requires a number of assumptions and conditional events to 

1356 come together in an unlikely manner for the results to be observed. I have assigned a probability 

1357 value of 0.01 to the results given this proposition to illustrate ‘low’.

1358

1359 d) In summary, both activities might be expected to give rise to the findings in this case. However, when 

1360 considering the probability of observing the results under the different propositions, the results would be 

1361 considered much more likely to be observed if the first proposition were true rather than the second. 

1362 Using the numerical example to express this outcome is to say that the evidence is in the order of 90 times 

1363 more likely given that Mr. X was driving the car rather than an unknown person.

1364

1365 Case Scenario 5

1366 Information: 

1367 A female, Ms. Y, is assaulted by a man as she exits a bar. She puts up a fight and scratches his face with 

1368 her dominant hand. The incident was observed by other customers. Mr. X states he was in the bar on the 

1369 night in question but did not assault the complainant. 

1370

1371 Question: Did Ms. Y scratch Mr. X or some unknown person?

1372 Propositions considered:

1373 ● Ms. Y scratched Mr. X outside the bar they both visited during the evening. 

1374 ● Ms. Y scratched an unknown person but was present in the same public space as Mr. X

1375

1376 Laboratory testing: 

1377 Samples are recovered from underneath Ms. Y’s fingernails, an hour after the incident. DNA from her 

1378 dominant hand yielded a mixed DNA profile exhibiting one major male component profile (Contributor 
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1379 1: mixture proportion 90%) and a minor profile (Contributor 2: mixture proportion 10%). The minor 

1380 profile contribution is concordant with the profile of Ms. Y.

1381 A statistical evaluation, a determines that the DNA profile result is on the order of 1 billion times more 

1382 likely if it originated from Ms. Y and Mr. X rather than Ms. Y and an unknown person who is unrelated 

1383 to Mr. X.

1384

1385 Assumptions: 

1386 a) Ms. Y is the minor contributor on the sample from her fingernails.

1387 b) There is a reasonable basis for concluding that Mr. X (rather than someone else) and Ms. Y contributed 

1388 to the DNA mixture.

1389 -Mr. X has given a statement that he was at the same bar at the same time as Ms. Y.

1390 -Mr. X matches the major contributor to the DNA profile from the fingernails of Ms. Y.

1391 -There is a large LR for the DNA profile that supports the inclusion of Mr. X.

1392 c) The evaluation considering activity level propositions is based upon the DNA from the fingernails of 

1393 Ms. Y coming from both Ms. Y and Mr. X.

1394 d) It is not in dispute that Ms. Y was involved in an incident where she scratched someone.

1395

1396 Oral Testimony:

1397 I have evaluated the DNA results in this case with respect to two propositions: either Ms. Y scratched Mr. 

1398 X during a scuffle, or she scratched some unknown person and Ms. Y and Mr. X just happened to be in 

1399 the same bar at the same time, which might have resulted in casual social interaction. 

1400

1401 There is data from research that discusses the occurrence of foreign DNA under one’s fingernails after 

1402 casual social contact. These studies show that recovering even a partial foreign profile from casual contact 

1403 is remote, and if it is, then it is expected to be at a very low level. Generally, foreign profiles can be 

1404 attributed to the person’s partner, which means it is very unusual to recover a major profile of a non1405 partner from someone’s fingernails, which is the data in this case. 

1406

1407 In contrast, scratching during a fight often leads to skin cells, and sometimes even blood and tissue, under 

1408 one’s nails. We can typically expect to recover good quality DNA profiles in this situation, often with the 

1409 foreign profile being the major donor. Studies done where volunteers have participated in vigorous 

1410 scratching experiments show foreign profiles in about 60% of the samples collected. Therefore, I have 

1411 assigned a probability to this evidence of 0.6 if the proposition that Ms. Y scratched Mr. X during a 

1412 scuffle was true. I would describe this probability as reasonable, because it is unlikely that the volunteers 

1413 in the scratching experiments I have described scratched as aggressively as what might happen during a 

1414 fight.

1415

1416 I have also assigned a probability to the evidence given the alternative proposition. If Ms. Y scratched an 

1417 unknown person instead of Mr. X, yet she was present in the same public space as Mr. X, then this means 

1418 we did not detect the DNA of the person she scratched. Based in part on the timely collection of the 

1419 sample, it is very unlikely to not detect DNA from the person Ms. Y scratched and to detect the DNA of a 

1420 single random person that happened to be at the same bar. Therefore, my probability of the DNA result is 

1421 very small, in the order of 0.0004 if this alternate proposition were true. 

1422

1423 When I consider the two propositions, the DNA results are 1500 times more likely if Ms. Y scratched Mr. 

1424 X rather than if Ms. Y scratched an unknown person but was present in the same public space as Mr. X.

1425

1426 It is important to note that the LR of 1 billion is for the evaluation of the DNA evidence if Mr. X were a 

1427 contributor or if some other unrelated person were a contributor of the material from under the fingernails 

1428 of Ms. Y. This LR of 1500 only relates to my interpretation of the evidence given scratching occurred 

1429 rather than if only social interaction took place. These numbers are not interchangeable and have very 
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1430 different magnitudes. We do not ever expect such large LRs for the evaluation of DNA results given 

1431 activity level propositions.

1432

1433 Evaluation:

1434 a) Background information:

1435 ● The scientific literature provides some useful information which allows a quantitative evaluation 

1436 of obtaining the results if DNA had been transferred by scratching or by social contact. Given that 

1437 the data represents the state of knowledge in this area at the moment, it can be used to inform my 

1438 expert opinion. 

1439 ● Research and data demonstrate that:

1440 ○ the incidence of foreign DNA beneath the fingernails in the general population as a result 

1441 of casual social contact is low; 

1442 ○ even if foreign DNA is detected, then invariably it is attributable to a person’s partner 

1443 rather than to a random individual and is generally attributed to more intimate contact;

1444 ○ foreign DNA profiles from casual social contact tend to be at a low/trace level;

1445 ○ scratching promotes the transfer of foreign DNA to fingernails resulting in good quality 

1446 foreign DNA profiles.

1447 ● Experience shows that transfer events of DNA in a social environment are typically complex and 

1448 involve direct and indirect DNA transfer involving many individuals. This situation is expected to 

1449 generate low level DNA profile results most likely from multiple foreign contributors.

1450

1451 b) Considering the first proposition:

1452 ● If Ms. Y had scratched Mr. X, then I expect a good quality DNA profile a little more than half of 

1453 the time when sampled in a timely fashion. This is based on a study where volunteers scratched 

1454 one another for time intervals from 5 to 30 seconds. In that study, 52% of the samples yielded a 

1455 complete (but minor) foreign DNA profile [Meandhim, 2040]. 

1456 ● More vigorous scratching, such as that which might occur during a fight, would be expected to 

1457 promote DNA transfer through the transfer of cellular material/skin tissue (and possibly blood) to 

1458 the fingernail area. The transferred cellular material would produce a good quality DNA profile 

1459 from the scratched person. The presence of Ms. Y’s own DNA would also be expected to form 

1460 part of the result. These expectations match the DNA findings in this case.

1461 ● However, I am adjusting the probability of these results slightly upward given the scratching 

1462 proposition, since this real-life situation resulted in a major foreign DNA profile. The study by 

1463 Meandhim only found minor profiles. This is support for more transfer in a real-life fight than 

1464 that found in the study. Therefore, in my opinion the probability of observing the results if she 

1465 scratched him would be in the order of 60%. 

1466

1467 c) Considering the alternative proposition:

1468 ● If no scratching has occurred between Ms. Y and Mr. X, then any DNA matching Mr. X has 

1469 arisen through social activity. 

1470 ● Therefore, the scratching has occurred between some unknown person and Ms. Y. This unknown 

1471 person’s DNA profile would be expected to be observed in the result, but this was not the case. 

1472 Literature shows the absence of a DNA profile of a person who has been scratched is in the order 

1473 of 20% [Soandso, 2050]. This value is favorable to the alternative proposition, as it is 

1474 understandable that volunteers would not agree to be scratched in an overly aggressive fashion. 

1475 The actual probability of not finding DNA from someone scratched in an aggressive fashion, and 

1476 collected in a timely fashion, is assumed to be less than 20%.

1477 ● Research data indicates that finding a foreign DNA profile, following social activities, associated 

1478 with someone’s fingernails is remote, in the order of 2% [Thisandthat, 2060].

1479 ● Additionally, my expectation is that any DNA profile resulting from social interaction would be a

1480 complex low-level mixture of multiple foreign contributors. Mr. X’s DNA would be at the same 
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1481 level as others whose DNA might have been transferred in a social environment. However, these 

1482 were not the findings in this case. Because a mixture with a major component was recovered 

1483 instead of a mixture with multiple trace contributors, the probability is adjusted downward by a 

1484 factor of 10 for transfer only via social interaction at the same bar. 

1485 • I have considered that the events are independent. Therefore, in my opinion, the probability of 

1486 observing the results if this proposition was true would be ‘very low’ on the order of 0.0004 (0.2 

1487 x 0.02 x 0.1 = 0.0004). 

1488

1489 d) In summary, both scenarios might give rise to the findings in this case. However, when considering the 

1490 probability of observing the results under the different propositions, the results would be considered 1500 

1491 times (0.6 / 0.0004) more likely to be observed if the first proposition were true rather than the second. 

1492

1493 Possible follow-up questions based on Scenario 5:

1494 Note: The previous examples conclude with the expert opinion. In practice, a series of follow-up 

1495 questions is common by both parties. The following are examples of questions that might arise after the 

1496 expert gives his or her opinion.

1497 

1498 Q1) So you are saying that it is a billion times more likely that Ms. Y scratched Mr. X?

1499 A) No. The statistical evaluation of a DNA profile result, which addresses the question regarding ‘whose 

1500 DNA is it?’ is an entirely different question from addressing ‘how and when did the DNA get there?’ This 

1501 means that the statistic of ‘1 billion’ has no bearing on the issues regarding how or when the DNA was 

1502 transferred. With respect to this question of scratching I have conducted a qualitative evaluation and given 

1503 a verbal likelihood ratio.

1504

1505 (Notes: This question crosses levels of the hierarchy of propositions and additionally, it contains a 

1506 transposed conditional.)

1507

1508 Q2) So what you are saying is that Ms. Y scratched Mr. X?

1509 A) No, this is a statement on what has happened, which I do not know. But, I can help with this issue by 

1510 assessing how likely the results are given the propositions. What I can say is, given that they were in the 

1511 same bar, I am offering my opinion on the probability of observing the mixed DNA profile result with 

1512 respect to two propositions: 

1513 either Ms. Y scratched Mr. X

1514 or, Ms. Y scratched some unknown person. 

1515 In my opinion the DNA result is more likely if Ms. Y scratched Mr. X than if Ms. Y scratched some 

1516 unknown person. This is a piece of the puzzle that must fit with the other puzzle pieces as the jury tries to 

1517 decide what happened. I do not have those other pieces.

1518

1519 (Notes: This question is asking for a direct comment on the activity that is alleged to have occurred. This 

1520 is sometimes referred to as “the ultimate issue”. It is not the role of the expert to comment on this. The 

1521 expert is to comment on the evidence, not the activity.)

1522

1523 Q3) So, it is possible the result can be explained by DNA transfer through social contact? 

1524 A) Yes, and I have indeed considered this explicitly in the evaluation of my results. But my evaluation 

1525 considers the probability of the DNA results (quantity, major corresponding to Mr. X) given one 

1526 proposition (here, scratching during a scuffle) compared to the alternative (here, social contact in the bar). 

1527 The evidence I evaluated shows a foreign male profile matching Mr. X from the nails of Ms. Y. I do not 

1528 expect this result from social contact. I do have some level of expectation of finding these results if 

1529 someone was scratched during a fight. Therefore, in my opinion the evidence is more likely if Ms. Y 

1530 scratched Mr. X than if Ms. Y scratched some unknown person. What is important is that my evaluation 

1531 does not eliminate the possibility that the results were obtained by social contact; however, given that 
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1532 alternative scenario to scratching during a fight, I consider these results less likely to be observed if the 

1533 social transfer scenario were true rather than if scratching had occurred as alleged.

1534

1535 (Notes: An LR which says the evidence is more likely if A than if B does not mean that B cannot be 

1536 “possible”. But the term possible has no meaning in a probabilistic assessment of the evidence. In this 

1537 example, the evidence is “possible” if either option occurred, but the expert analysis clearly favors the 

1538 evidence as being more likely if Mr. X were scratched by Ms. Y.)

1539

1540 Q4) Are there any other explanations for the findings?

1541

1542 A) I do not have any additional information as a DNA scientist that would help me say what could have 

1543 happened in this case. I can give you my opinion on the results when given activities of interest to the 

1544 court, but not on the (alleged) activities themselves. It is not my role. In addition, discussing the 

1545 probability of these results given only a single explanation offers no real value, as it needs to be put into 

1546 contrast using alternate propositions. Only then may I offer an opinion that the evidence is more likely 

1547 under proposition A than proposition B, and those propositions must have some reasonable relation to the 

1548 case.

1549

1550 (Notes: The expert needs to be aware of the difference between an explanation after the results are known 

1551 compared to a proposition based on case information. The expert should not be the source of new 

1552 propositions, and any evaluation of a single proposition by itself has no real meaning in the context of the 

1553 activities alleged by the parties. For example, the probability of the evidence given explanation A is Pr(E | 

1554 A) = 0.001 (one in one thousand). Numerically, that is a low number. However if the probability of the 

1555 evidence given another explanation (B) is Pr(E | B) = 0.0001 (one in ten thousand), then comparatively, 

1556 Pr(E | A) is a high probability. )

1557

1558 Q5) Your opinion is subjective in that it is based on personal beliefs. Why should the court rely on this?

1559 A) I have considered my results in the light of two propositions. For this I have used my personal 

1560 knowledge and in that sense, it is my opinion, so it is subjective. But it is not arbitrary nor biased. My 

1561 opinion is based on my experience, my expertise, and the relevant data and literature that addresses these 

1562 questions. I am qualified, by virtue of my training, to address the questions put forward to me by the court 

1563 following a recognized framework, which accommodates both the prosecution and defense alternatives. 

1564 My opinion is presented in a transparent manner which allows scrutiny and discussion.

1565

1566 (Notes: All expert opinion is subjective to one degree or another, or else there would be no expertise. 

1567 Even evaluation of evidence given the sub-source level propositions is subjective, as a choice was made 

1568 about which allele frequency database to use, artifact labeling, statistical models or software, and other 

1569 issues.)

1570

1571 Q6) The LR reported for the activity level propositions is very much lower than that reported when 

1572 considering whose DNA might be present (1 billion). Doesn’t this mean that the LR of 1500 has less 

1573 probative value?

1574 A) First of all, the two LRs relate to two very different questions. The LR of “1 billion” is helping to 

1575 answer the question of “Whose DNA is present?” and the LR of 1500 is helping to answer the question 

1576 “How did the DNA get there?” The factors that go into the calculation of these two different numbers 

1577 consider very different things with different magnitudes. The results are also very different. In the first 

1578 LR, the results pertain to the comparison of two DNA profiles. In the second, as there is no dispute on the 

1579 source of the DNA for the activity propositions, the results are only the quantity of DNA from which 

1580 donors - or lack of donors. The two numbers are therefore not comparable as they are addressing different 

1581 things. 

1582
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1583

1584

1585 Annex C

1586 (informative)

1587 Bibliography

1588 van den Berge M, Ozcanhan G, Zijlstra S, Lindenbergh A, Sijen T. Prevalence of human cell material: 

1589 DNA and RNA profiling of public and private objects and after activity scenarios. Forensic Science 

1590 International: Genetics 21 (2016) 81-89.

1591 Bowman ZE, Mosse KSA, Sungaila AM, van Oorschot RAH, Hartman D. Detection of offender DNA 

1592 following skin-to-skin contact with a victim. Forensic Science International: Genetics 37 (2018) 252-259.

1593 Boyko T, Szkuta B, Mitchell RJ, van Oorschot RAH. Prevalence of DNA from the driver, passengers 

1594 and others within a car of an exclusive driver. Forensic Science International 307 (2020) 1103139.

1595 Brayley-Morris H, Sorrell A, Revoir AP, Meakin GE, Syndercombe Court D, Morgan RM. Persistence 

1596 of DNA from laundered semen stains: Implications for child sex trafficking cases. Forensic Science 

1597 International: Genetics 19 (2015) 165-171.

1598 Breathnach M, Williams L, McKenna L, Moore E. Probability of detection of DNA deposited by habitual 

1599 wearer and/or the second individual who touched the garment. Forensic Science International: Genetics 

1600 20 (2016) 53-60.

1601 Buckingham AK, Harvey ML, van Oorschot RAH. The origin of unknown source DNA from touched 

1602 objects. Forensic Science International: Genetics 25 (2016) 26-33.

1603 Butcher EV, van Oorschot RAH, Morgan RM, Meakin GE. Opportunistic crimes: Evaluation of DNA 

1604 from regularly used knives after a brief use by a different person. Forensic Science International: 

1605 Genetics 42 (2019) 135-140.

1606 Cook O, Dixon L. The prevalence of mixed DNA profiles in fingernail samples taken from individuals in 

1607 the general population. Forensic Science International: Genetics 1(1) (2007) 62-68.

1608 Dowlman EA, Martin NC, Foy MJ, Lochner T, Neocleous T. The prevalence of mixed DNA profiles on 

1609 fingernail swabs. Science & Justice 50(2) (2010) 64-71.

1610 Flanagan N, McAlister C. The transfer and persistence of DNA under the fingernails following digital 

1611 penetration of the vagina. Forensic Science International: Genetics 5(5) (2011) 479-483.

1612 Fonnelop AE, Johannessen H, Heen G, Molland K, Gill P. A retrospective study on the transfer, 

1613 persistence and recovery of sperm and epithelial cells in samples collected in sexual assault casework. 

1614 Forensic Science International: Genetics 41 (2019) 102153.

1615 Goray M, Eken E, Mitchell RJ, van Oorschot RAH. Secondary DNA transfer of biological substances 

1616 under varying test conditions. Forensic Science International: Genetics 4(2) (2010) 62-67.

1617 Goray M, Kokshoorn B, Steensma K, Szkuta B, van Oorschot RAH. DNA detection of a temporary and 

1618 original user of an office space. Forensic Science International: Genetics 44 (2020) 102203.
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1619 Gosch A, Euteneuer J, Preuss-Wössner J, Courts C. DNA transfer to firearms in alternative realistic 

1620 handling scenarios. Forensic Science International: Genetics 48 (2020) 102355.

1621 Graham EAM, Rutty GN. Investigation into “normal” background DNA on adult necks: Implications for 

1622 DNA profiling of manual strangulation victims. Journal of Forensic Sciences 53(5) (2008) 1074-1082.

1623 Kamodyová N, Durdiaková J, Celec P, Sedláčková T, Repiská G, Sviežená B, Minárik G. Prevalence and 

1624 persistence of male DNA identified in mixed saliva samples after intense kissing. Forensic Science 

1625 International: Genetics 7(1) (2013) 124-128.

1626 Kenna J, Smyth M, McKenna L, Dockery C, McDermott SD. The recovery and persistence of salivary 

1627 DNA on human skin. Journal of Forensic Sciences 56(1) (2011) 170-175.

1628 Kokshoorn B, Aarts LHJ, Ansell R, Connolly E, Drotz W, Kloosterman AD, McKenna LG, Szkuta B, 

1629 and van Oorschot RAH. Sharing data on DNA transfer, persistence, prevalence and recovery: Arguments 

1630 for harmonization and standardization. Forensic Science International: Genetics 37 (2018) 260-269.

1631 Lowe A, Murray C, Whitaker J, Tully G, Gill P. The propensity of individuals to deposit DNA and 

1632 secondary transfer of low level DNA from individuals to inert surfaces. Forensic Science International 

1633 129(1) (2002) 25-34.

1634 Magee AM, Breathnach M, Doak S, Thornton F, Noone C, McKenna LG. Wearer and non-wearer DNA 

1635 on the collars and cuffs of upper garments of worn clothing. Forensic Science International: Genetics 34 

1636 (2018) 152-161.

1637 Malsom S, Flanagan N, McAlister C, Dixon L. The prevalence of mixed DNA profiles in fingernail 

1638 samples taken from couples who co-habit using autosomal and Y-STRs. Forensic Science International: 

1639 Genetics 3(2) (2009) 57-62.

1640 Matte M, Williams L, Frappier R, Newman J. Prevalence and persistence of foreign DNA beneath 

1641 fingernails. Forensic Science International: Genetics 6(2) (2012) 236-243.

1642 Meakin GE, Butcher EV, van Oorschot RAH, Morgan RM. Trace DNA evidence dynamics: An 

1643 investigation into the deposition and persistence of directly and indirectly transferred DNA on regularly 

1644 used knives. Forensic Science International: Genetics 29 (2017) 38-47.

1645

1646 Murphy C, Kenna J, Flanagan L, Gorman ML, Boland C, Ryan J. A study of the background levels of 

1647 male DNA on underpants worn by females. Journal of Forensic Sciences 65(2) (2020) 399-405.

1648 Oldoni F, Castella V, Hall D. Shedding light on the relative DNA contribution of two persons handling 

1649 the same object. Forensic Science International: Genetics 24 (2016) 148-157.

1650 van Oorschot RAH, Glavich G, Mitchell RJ. Persistence of DNA deposited by the original user on 

1651 objects after subsequent use by a second person. Forensic Science International: Genetics 8(1) (2014) 

1652 219-225.

1653 Otten L, Banken S, Schürenkamp M, Schulze-Johann K, Sibbing U, Pfeiffer H, Vennemann M. 

1654 Secondary DNA transfer by working gloves. Forensic Science International: Genetics 43 (2019) 102126.

1655 Ramos P, Handt O, Taylor D. Investigating the position and level of DNA transfer to undergarments 

1656 during digital sexual assault. Forensic Science International: Genetics 47 (2020) 102316.
OSAC 2022-S-0024 Best Practice Recommendations

for Evaluative Forensic DNA Testimony

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1657 Raymond JJ, van Oorschot RAH, Gunn PR, Walsh SJ, Roux C. Trace evidence characteristics of DNA: 

1658 A preliminary investigation of the persistence of DNA at crime scenes. Forensic Science International: 

1659 Genetics 4(1) (2009) 26-33.

1660 Samie L, Hicks T, Castella V, Taroni F. Stabbing simulations and DNA transfer. Forensic Science 

1661 International: Genetics 22 (2016) 73-80.

1662 Samie L, Champod C, Taylor D, Taroni F. The use of Bayesian networks and simulation methods to 

1663 identify the variables impacting the value of evidence assessed under activity level propositions in 

1664 stabbing cases. Forensic Science International: Genetics 48 (2020) 102334.

1665 Steensma K, Ansell R, Clarisse L, Connolly E, Kloosterman AD, McKenna LG, van Oorschot RAH, 

1666 Szkuta B, Kokshoorn B. An inter-laboratory comparison study on transfer, persistence and recovery of 

1667 DNA from cable ties. Forensic Science International: Genetics 31 (2017) 95-104.

1668 Szkuta B, Ballantyne KN, van Oorschot RAH. Transfer and persistence of DNA on the hands and the 

1669 influence of activities performed. Forensic Science International: Genetics 28 (2017) 10-20.

1670 Szkuta B, Ballantyne KN, Kokshoorn B, van Oorschot RAH. Transfer and persistence of non-self DNA 

1671 on hands over time: Using empirical data to evaluate DNA evidence given activity level propositions. 

1672 Forensic Science International: Genetics 33 (2018) 84-97.

1673 Szkuta B, Ansell R, Boiso L, Connolly E, Kloosterman AD, Kokshoorn B, McKenna LG, Steensma K, 

1674 and van Oorschot RAH. Assessment of the transfer, persistence, prevalence and recovery of DNA traces 

1675 from clothing: An inter-laboratory study on worn upper garments. Forensic Science International: 

1676 Genetics 42 (2019) 56-68.

1677 Szkuta B, Ansell R, Boiso L, Connolly E, Kloosterman AD, Kokshoorn B, McKenna LG, Steensma K, 

1678 van Oorschot RAH. DNA transfer to worn upper garments during different activities and contacts: An 

1679 inter-laboratory study. Forensic Science International: Genetics 46 (2020) 102268.

1680 Taroni F, Biedermann A, Vuille J, Morling N. Whose DNA is this? How relevant a question? (a note for 

1681 forensic scientists). Forensic Science International: Genetics 7(4) (2013) 467-470.

1682 Taylor D, Bright JA, Buckleton J. The ‘factor of two’ issue in mixed DNA profiles. Journal of Theoretical 

1683 Biology 363 (2014) 300-306.

1684

1685 Taylor D, Abarno D, Hicks T, Champod C. Evaluating forensic biology results given source level 

1686 propositions. Forensic Science International: Genetics 21 (2016) 54-67.

1687 Taylor D, Abarno D, Rowe E, Rask-Nielsen L. Observations of DNA transfer within an operational 

1688 Forensic Biology Laboratory. Forensic Science International: Genetics 23 (2016) 33-49.

1689 Taylor D, Biedermann A, Samie L, Pun KM, Hicks T, Champod C. Helping to distinguish primary from 

1690 secondary transfer events for trace DNA. Forensic Science International: Genetics 28 (2017) 155-177.

1691 Taylor D, Kokshoorn B, Biedermann A. Evaluation of forensic genetics findings given activity level 

1692 propositions: A review. Forensic Science International: Genetics 36 (2018) 34-49.
OSAC 2022-S-0024 Best Practice Recommendations

for Evaluative Forensic DNA Testimony

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1693 Taylor D, Biedermann A, Hicks T, Champod C. A template for constructing Bayesian networks in 

1694 forensic biology cases when considering activity level propositions. Forensic Science International: 

1695 Genetics 33 (2018) 136-146.

1696 Taylor D, Samie L, Champod C. Using Bayesian networks to track DNA movement through complex 

1697 transfer scenarios. Forensic Science International: Genetics 42 (2019) 69-80.

1698 Verdon TJ, Mitchell RJ, van Oorschot RAH. The influence of substrate on DNA transfer and extraction 

1699 efficiency. Forensic Science International: Genetics 7(1) (2013) 167-175.

1700 Williams GA, Maskell PD. Embracing likelihood ratios and highlighting the principles of forensic 

1701 interpretation. Forensic Science International: Reports 3 (2021) 100209.