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Standard for Verification of Source Conclusions in Toolmark Examinations

Standard for Verification 

of Source Conclusions in 

Toolmark Examinations

Firearms & Toolmarks Subcommittee

Physics/Pattern Interpretation Scientific Area Committee

Organization of Scientific Area Committees (OSAC) for Forensic Science
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Standard for Verification of Source Conclusions in Toolmark Examinations

OSAC Proposed Standard

Standard for Verification of Source 

Conclusions in Toolmark 

Examinations

Prepared by 

Firearms & Toolmarks Subcommittee

Version: 1.0

August 2020

Disclaimer:

This document has been developed by the Firearms & Toolmarks Subcommittee of the 

Organization of Scientific Area Committees (OSAC) for Forensic Science through a consensus 

process and is proposed for further development through a Standard Developing Organization 

(SDO). This document is being made available so that the forensic science community and 

interested parties can consider the recommendations of the OSAC pertaining to applicable 

forensic science practices. The document was developed with input from experts in a broad array 

of forensic science disciplines as well as scientific research, measurement science, statistics, law, 

and policy. 

This document has not been published by an SDO. Its contents are subject to change during the 

standards development process. All interested groups or individuals are strongly encouraged to 

submit comments on this proposed document during the open comment period administered by 

the Academy Standards Board (ASB) https://www.asbstandardsboard.org/. 
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Standard for Verification of Source Conclusions in Toolmark Examinations

Keywords: bias, comparison, primary examiner, verification

This document was developed to provide a standard for the verification of toolmark source 

conclusions by a second qualified examiner. 

Foreword

This standard was proposed by the Firearms and Toolmarks Subcommittee of the 

Organization of Scientific Area Committees (OSAC) by submitting a request to the American 

Academy of Forensic Sciences (AAFS) Academy Standards Board (ASB). This document is 

intended to provide a standard for the verification of toolmark source conclusions by a 

second qualified examiner. This document takes into consideration the current state of 

professional practices and scientific research on contextual bias and confirmation bias1.

Documents that contain information related to this standard include:

● Standard Scale of Source Conclusions and Criteria for Toolmark Examinations

● Minimum Education Requirements for Firearm and Toolmark Examiner Trainees

● Requirements and Recommendations for a Firearm and Toolmark Examiner Training 

Program

1 Numerous scientific studies, both inside and outside of forensic science, have provided convincing evidence 

for the existence of contextual influences that may have a deleterious effect on the judgement of examiners 

performing Peer Reviews. Some of these studies that relate directly to forensic science issues are listed in the 

Bibliography. 
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Standard for Verification of Source Conclusions in Toolmark Examinations

Acknowledgements

Editor:

Deputy Editor(s):

Drafting Working Group Members:

Eric Collins Contra Costa County Office of the Sheriff

Gerard Dutton Tasmania Police, Australia

Michael Haag Albuquerque Police Department

Nancy McCombs California Department of Justice

Max Morris Iowa State University

Rebecca Mullen Arkansas State Crime Laboratory

John Murdock John Murdock & Associates – Forensic Consultants

Michael Neel Bureau of Alcohol, Tobacco, Firearms and Explosives

Ronald Nichols Nichols Forensic Consulting

Gerhard Wevers Institute of Environmental Science and Research, New Zealand

Consensus Group Members:
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Standard for Verification of Source Conclusions in Toolmark Examinations

Table of Contents

1 Scope 6

2 6

3 6

4 7

4.1 7

4.2 7
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Standard for Verification of Source Conclusions in Toolmark Examinations

1 Scope

This standard provides a standard for conducting verifications of source conclusions arising from 

forensic toolmark comparisons. This document is limited to the process of verifying the source 

conclusions reached by the primary firearm and toolmark examiner in a case and does not address 

or consider other types of technical casework review. 

2 Normative References

3 Terms and Definitions

For the purposes of this document, the following definitions and abbreviations apply:

3.1

Forensic Science Services Provider (FSSP)

A forensic science agency or forensic science practitioner providing forensic science services

3.2

Light Comparison Microscopy (LCM)

A method of toolmark analysis involving the use of a brightfield microscope to allow side-by-side 

comparison.

3.3

Primary Examiner

The qualified firearm and toolmark examiner responsible for conducting a toolmark examination, 

making source conclusions, and authoring a report.

3.4

Comparison

The side-by-side microscopic examination of two items or two toolmarks.

3.5

Verification

The independent comparison of previously compared toolmarks to provide a quality check of a 

source conclusion. 

3.6

Verifier

The qualified firearm and toolmark examiner tasked with reaching an independent source

conclusion regarding evidence examined by the primary examiner.

3.7

Virtual Comparison Microscopy (VCM)

A method of toolmark analysis involving the use of hardware and software to allow side-by-side 

comparison of 3D topography data.
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Standard for Verification of Source Conclusions in Toolmark Examinations

4 Requirements

4.1 Extent of Verification

All (100%) of the primary examiner’s source conclusions shall be subjected to the verification 

process. 

4.2 Method

The FSSP shall implement written procedures for verification. All verifications should be conducted 

by examining the original evidence using LCM or VCM whenever practicable.

However, it is recognized that there are circumstances in which verification by an independent 

examiner viewing the original evidence may not be possible or practically feasible. In these 

circumstances, and when sufficient agreement or disagreement of class or individual characteristics 

can be clearly demonstrated to support an opinion of Exclusion or Identification, photographic 

and/or digital images may be used by the verifier to form an opinion. The representation or 

information provided to the verifier, including additional documentation from the primary 

examiner, must be sufficient, in the verifier’s opinion, to support an independent source opinion. 

Due to the numerous variables that can lead to inconclusive comparison opinions, it is not feasible 

to verify these types of comparisons through the use of photographs only. Therefore, all 

inconclusive comparison results made by the primary examiner shall be verified by examining the 

original evidence using LCM or VCM.

The verifier shall not be informed of the primary examiner’s source opinion(s) nor be exposed to 

task-irrelevant information prior to reaching their own opinion(s). It is recognized that this may 

not be feasible if verification is performed using photographs of comparisons taken by the primary 

examiner. 

4.3 Assignment of Verifier

The FSSP should implement policies for the assignment of verifiers. Whenever practicable, the 

primary examiner shall not assign their own verifier. Assignment may be accomplished through a 

written procedure or less formally, depending on factors such as staff size and availability, potential 

for confirmation bias by particular staff members due to known exposure to task-irrelevant case 

information, or other needs and requirements of the FSSP. When assigning a verifier, the FSSP will 

make every effort to select an examiner who has no prior knowledge of the primary examiner’s 

source opinions.

4.4 Item Identity Check

 As with all casework, it is the responsibility of the primary examiner to ensure that the compared 

items are correctly controlled. It is the primary examiner’s responsibility to present the correct 

items to the verifier for the verification process. However, it is also incumbent on the verifier to 

ensure the proper items have been compared during their verification. Therefore, the FSSP shall
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Standard for Verification of Source Conclusions in Toolmark Examinations

have a written procedure for ensuring compared items are correctly controlled, to include securely 

marking the evidence items (whenever practicable) prior to comparison and having the verifier 

deliberately check the identity of each item immediately before or after they are microscopically 

verified.

4.5 Resolution of Conflicting Conclusion(s) between Primary Examiner and Verifier

The FSSP shall have a policy for the arbitration of differences in source conclusion(s) between the 

primary examiner and verifier. Differences in conclusions may be resolved through a consultation 

between the conflicting examiners, or, in the case of an impasse, it may require a review by a third 

qualified examiner. The third examiner can decide in favor of either opinion or send the evidence 

to another FSSP for arbitration, if needed. The third examiner shall not be informed of the 

conclusions reached by the primary examiner and verifier before making their assessment. The 

person acting as the third examiner should not be chosen by either the primary examiner or 

verifier. The FSSP’s policy shall address how the third examiner is selected. If necessary, the third 

examiner can be a qualified examiner from another FSSP.

4.6 Documentation of Verifications

The following information shall be documented:

● The identity of the verifier 

● The date(s) of verification

● The basis for the verifier’s opinion (e.g. what marks were compared) 

● The verifier’s conclusion(s) 

● Affirmation of the verifier’s item identity check 

● The method of review (e.g., LCM, VCM, or photographs) 

● Any disagreement of source opinions and their resolutions, including any change(s) to 

original conclusion(s)

Bibliography

1. Dror, I., 2013. The Ambition to be Scientific: Human Expert Performance and 

Objectivity. Science and Justice 53: 81-82.

2. Dror, I., 2013. Practical solutions to cognitive and human factor challenges in 

forensic science. Forensic Science Policy and Management 4: 105-113.

3. Kerstholt, J., et al, 2016. Does suggestive information cause a confirmation bias in 

bullet comparisons? Forensic Science International 198: 138-142.

4. Mattijssen, E., et al, 2015. Implementing context information management in 

forensic casework: Minimizing contextual bias in firearms examination. Science 

and Justice, http://dx.doi.org/10.1016/j.scijus.2015.11.004.