Sunday, May 1, 2011

Part IV of Fingerprinting Under the Microscope: Probative Value

Where do all these numbers leave us? Plainly, they demonstrate that the error rate for human examiners is not zero. They show that the examiners in the experimental conditions made false identifications at a rate of only 1 in a 1,000, and they made false exclusions at a rate exceeding 75 in 1,000. These error rates indicate that fingerprint identifications and exclusions are (or can be) valid. That is the Daubert question. A human being's use of the features that LPEs rely on in fingerprint comparisons is a valid means of identifying fingerprints.

But these numbers do not give the probative value of an identification or an exclusion in a particular case. For one thing, these numbers apply to a large group of examiners who are not representative of all LPEs and to challenging pairings of prints. To permit an extrapolation to the particular case, however, let us assume that the examiner in that case performed identically to this group in examining a pair of prints that was comparable to the pairs in the study. Suppose this LPE then made an identification to the defendant. How much support would that provide for the prosecution’s case?

The positive predictive value of 99.8% does not apply to this case unless the prior probability that the latent and the exemplar are mates is the same as the prevalence of matees in the study. The prevalence of mates among the pairs found to be of value for individualization was 4083/5969 = 68.4%. The prior probability here could be lower (if the exemplar came from an AFIS search and there were no other evidence in the case, for example). Or it could be higher (if the other evidence was such that the probability of a mate without regard to the match exceeded 68.4%).

The expert witness is not in a position to choose prior probabilities for the jurors, but an expert could display the posterior probability as a function of the possible priors. A figure in the Noblis-FBI study graphs this function, and it might be presented to the jury. This method of explaining a match has been debated in the legal literature for some 40 years. See David H. Kaye et al., The New Wigmore on Evidence: Expert Evidence (2d ed. 2011).

Instead of talking in terms of the posterior probability, one can focus on the likelihood ratio (LR) in its own right. This ratio indicates how much the finding of a match shifts the odds (based on other evidence) that the defendant is the source of the latent print. The LR is given by
P(identification | mate & VIn & conclusion)
P(identification | nonmate & VIn & conclusion)
= Sensitivity
(1 – Specificity)
= 89.1
(1 – 99.8)
= 587.
An expert might testify that, based on this study, the identification is strong evidence for the prosecution because it is hundreds of times more likely to arise when a latent print and an exemplar originate from the same finger than when they come from different sources. The defense would have to argue that the examiner is below the average of the study group in skill, that he might have taken less care than those LPEs did, that he had more difficult prints to work with, or that even with this finding, the prosecution case leaves a reasonable doubt. If there were blind verification of the examiner's conclusion, the prosecutor could counter that the evidence is even more compelling than the LR of 587 suggests. The jury might regard the fingerprint evidence as powerful but not conclusive. With a careful examiner, that would appear to be a reasonable conclusion.

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