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1 Addendum to the PCAST Report on Forensic Science in Criminal Courts 1 (2017)

handle is hein.prescomm/adforsc0001 and id is 1 raw text is: A1 ADDENDUM TO THE PCAST REPORI ON FORENSIC SCIENC r CRiMiNAL 0 RS On September 20, 2016, PCAST released its unanimous report to the President entitled Forensic Science in Criminal Courts: Ensuring Scientific Validity of Feature-Comparison Methods. This new document, approved by PCAST on January 6, 2017, is an addendum to the earlier report developed to address input received from stakeholders in the intervening period. PCAST's 2016 report addressed the question of when expert testimony based on a forensic feature- comparison method should be deemed admissible in criminal courts.' We briefly summarize key aspects of the previous report. Forensic feature-comparison methods PCAST chose to focus solely on forensic feature-comparison methods. These methods seek to determine whether a questioned sample is likely to have come from a known source based on shared features in certain types of evidence. Specific methods are defined by such elements as: (i) the type of evidence examined (e.g., DNA, fingerprints, striations on bullets, bitemarks, footwear impressions, head-hair); (ii) the complexity of the sample examined (e.g., a DNA sample from a single person vs. a three- person mixture in which a person of interest may have contributed only 1%); and (iii) whether the conclusion concerns only class characteristics or individual characteristics (e.g., whether a shoeprint was made by a pair of size 12 Adidas Supernova Classic running shoes vs. whether it was made by a specific pair of such running shoes). The U.S. legal system recognizes that scientific methods can assist the quest for justice, by revealing information and allowing inferences that lie beyond the experience of ordinary observers. But, precisely because the conclusions are potentially so powerful and persuasive, the law requires scientific testimony be based on methods that are scientifically valid and reliable.2 Requirement for empirical testing of subiective methods In its report, PCAST noted that the only way to establish the scientific validity and degree of reliability of a subjective forensic feature-comparison method-that is, one involving significant human judgment-is to test it empirically by seeing how often examiners actually get the right answer. Such an empirical test of a subjective forensic feature-comparison method is referred to as a black-box test. The point reflects a central tenet underlying all science: an empirical claim cannot be considered scientifically valid until it has been empirically tested. If practitioners of a subjective forensic feature-comparison method claim that, through a procedure involving substantial human judgment, they can determine with reasonable accuracy whether a particular type of evidence came from a particular source (e.g., a specific type of pistol or a specific pistol), the claim cannot be considered scientifically valid and reliable until one has tested it by (i) providing an adequate number of examiners with an adequate number of test problems that resemble those found in forensic practice and (ii) determining whether they get the right answer with acceptable I As noted in the report, PCAST did not address the use of forensic methods in criminal investigations, as opposed to in criminal prosecution in courts. 2 See discussion of the Federal Rules of Evidence in Chapter 3 of PCAST's report.

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