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Scientific Validation of Mixture Interpretation Method

Perlin, M.W. Scientific validation of mixture interpretation method in the Proceedings of Promega's Seventeenth International Symposium on Human Identification. Nashville, TN, 2006.




For courtroom admissibility, Federal Rule of Evidence (FRE) 702 mandates the reliability of (a) data, (b) method, and (c) application of method to data. The "reliability" of each component is determined (according to jurisdiction) by the Frye (1923) or Daubert (1993) standard. Whereas Frye entails only general acceptance, Daubert also provides for a testable approach, whose error rate has been determined, and has been communicated by peer review dissemination. These Daubert criteria are typically met by conducting scientific validation studies that establish the reliability of an approach by testing it and determining an error rate.

Forensic STR data have undergone extremely rigorous scientific validation in this country, with validation studies of laboratory processes routinely introduced as courtroom evidence in order to establish admissibility. Similarly, the DNA science of interpreting and matching single source profiles is solidly grounded in the rigor of population genetics. Since there is only one correct designation of a pristine single source profile, concordance studies can compare (the theoretically identical) results of two different examiners.

However, interpretation of mixed DNA samples need not produce unambiguous results. Different laboratories follow different mixture interpretation guidelines. Moreover, different examiners within the same laboratory who are following the same guidelines often infer different STR profiles. Therefore, there is no concordance in current forensic practice on what constitutes a "correct" mixture solution. Thus, it is not possible to conduct a mixture interpretation concordance study in order to validate a mixture interpretation method. But it is essential to have some way of testing the reliability of mixture interpretation methods so that inferred profiles from DNA mixtures can be scientifically validated and admitted as legal evidence.

We have developed a general approach to scientifically validating mixed DNA profiles. Instead of conducting a concordance comparison (which is not possible), our approach determines the amount of information present in the DNA match between an inferred mixture profile and a reference profile. By examining these numerical measures of match information, one can assess the reliability of a mixture interpretation method.

We have tested our validation approach on mixed DNA samples from multiple crime laboratories, each of whom uses their own interpretation methods. We found that different interpretation methods produce different inferred profiles with varying match specificity. However, regardless of match information, once a lab's interpretation method has been scientifically validated, its inferred profiles (and DNA matches that include those profiles) become admissible as reliable evidence in court. In this paper, we present our novel approach to scientifically validating a lab's guidelines for interpreting DNA mixtures. We describe different mixture interpretation guidelines used in forensic practice, and show how these guidelines can all be validated as reliable methods. And we illustrate how our approach can be used for presenting DNA results in court. By scientifically validating the mixture interpretation method that it uses, a crime lab can go beyond just the admissibility of its validated DNA laboratory data, and also ensure the admissibility of its validated interpretation methods, inferred profiles and DNA matches.