[T]esting labs lacked validated and consistently-applied procedures ... for declaring whether two [DNA] patterns matched within a given tolerance, and for determining the probability of such matches arising by chance in the population (P. 2)
Here, a “match” is a correspondence in measurements, and it is plainly not synonymous with a proposed identification. The identification would be an inference from the matching measurements that could arise "by chance" or because the DNA samples being analyzed are from the same source.
By subjective methods, we mean methods including key procedures that involve significant human judgment—for example, about which features to select within a pattern or how to determine whether the features are sufficiently similar to be called a probable match. (P. 5 n.3)
Now it is seems that “match” refers to “sufficiently similar” features and to an identification of a single, probable source of the traces with these similar features.
Forensic examiners should therefore report findings of a proposed identification with clarity and restraint, explaining in each case that the fact that two samples satisfy a method’s criteria for a proposed match does not mean that the samples are from the same source. For example, if the false positive rate of a method has been found to be 1 in 50, experts should not imply that the method is able to produce results at a higher accuracy. (P. 6)
Here, “proposed match” seems to be equated to “proposed identification.” (Or does “proposed match” mean that the degree of similarity a method uses to characterize the measurements as matching might not really be present in the particular case, but is merely alleged to exist?)
Later, the report argues that
Is this a blanket recommendation to stop using the term “match” for an observed degree of similarity? It prompted the following rejoinder:Because the term “match” is likely to imply an inappropriately high probative value, a more neutral term should be used for an examiner’s belief that two samples come from the same source. We suggest the term “proposed identification” to appropriately convey the examiner’s conclusion ... . (Pp. 45-46.)
I doubt that it is either realistic or essential to banish the word “match” from the lexicon for identification evidence. But it is essential to be clear about its meaning. As one textbook on interpreting forensic-science evidence cautions:Most scientists would be comfortable with the notion of observing that two samples matched but would, rightly, refuse to take the logically unsupportable step of inferring that this observation amounts to an identification. 1/
NOTESYet another word that is the source of much confusion is 'match'. 'Match' can mean three different things:
• Two traces share some characteristic which we have defined and categorised, for example, when two fibres are both made of nylon.
• Two traces display characteristics which are on a continuous scale but fall within some arbitrarily defined distance of each other.
• Two traces have the same source, as implied in expressions such as 'probable match' or 'possible match'.
If the word 'match' must be used, it should be carefully defined. 2/
- I.W. Evett, C.E.H. Berger, J.S. Buckleton, C. Champod, & G. Jackson, Finding the Way Forward for Forensic Science in the US—A Commentary on the PCAST Report, 278 Forensic Sci. Int'l 16, 19 (2017). One might question whether “most scientists” should “be comfortable” with observing that two samples “matched” on a continuous variable (such as the medullary index of hair). Designating a range of matching and nonmatching values means that a close nonmatch is treated radically differently than an almost identical match at the edge of the matching zone. Ideally, a measure of probative value of evidence should not incorporate this discontinuity.
- Bernard Robertson, Charles E. H. Berger, and G. A. Vignaux, Interpreting Evidence: Evaluating Forensic Science in the Courtroom 63 (2d ed. 2016).
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