Opinion ID: 1160857
Heading Depth: 2
Heading Rank: 4

Heading: Use of Population Databases to Assess Significance of Match

Text: Once a match at multiple loci has been declared, the next step is to determine its statistical significance. ( Barney, supra, 8 Cal.App.4th at p. 809, 10 Cal.Rptr.2d 731.) Unless a nonmatch between any band of the suspect's DNA and the corresponding band of the questioned sample conclusively eliminates the suspect as the source of that sample, a match of one or more of the suspect's bands with those of the sample places the suspect within a class of persons from whom the sample could have originated. The fact finder's determination of guilt may then turn on the degree of probability that the suspect was indeed the source of the sample. That probability, however, will usually depend, not on the DNA findings alone, but on a combination of those findings together with other, non-DNA incriminating evidence. (See State v. Bloom (Minn.1994) 516 N.W.2d 159, 162-163.) The question properly addressed by the DNA analysis is therefore this: Given that the suspect's known sample has satisfied the match criteria, what is the probability that a person chosen at random from the relevant population would likewise have a DNA profile matching that of the evidentiary sample? [12] That probability is usually expressed as a fractioni.e., the probability that one out of a stated number of persons in the population (e.g., 1 out of 100,000) would match the DNA profile of the evidentiary sample in question. A greater probability, that is to say, a fraction with a smaller denominator (e.g., 1 out of 10,000), would tend to favor the suspect by increasing the probability that one or more other persons has a DNA profile matching the evidentiary sample. To assess the probability in question, the FBI and Cellmark [and the OCSD crime laboratory in this case] calculate how frequently each pair of bands produced by one probe is found in a target population. ( Barney, supra, 8 Cal. App.4th at p. 809, 10 Cal.Rptr.2d 731.) For this purpose, those and other forensic laboratories use one or more population databases containing measurements of the DNA fragments of several hundred persons at each of the loci reached by the probes. [13] The samples from which those measurements are derived come from such varied sources as blood banks, hospitals, clinics, genetics laboratories, and law enforcement personnel. (See 1996 NRC Rep., supra, p. 126.)