Forensic commentary series: Hypothesis testing in law and forensic science: A memorandum

This Forum Commentary series presents views on a letter from a group of lawyers and judges advising the Organization of Scientific Area Committees for Forensic Science (OSAC).1 In response to calls for improving the practices of forensic science,2 the National Institute of Standards and Technology (NIST) created the Scientific Area Committees in 2014 to promote and develop standards “that are fit-forpurpose and based on sound scientific principles.”3 The memorandum from the Legal Resource Committee (LRC)4 responds to a question from a scientist on OSAC’s governing board about whether the criminal law’s concern with avoiding false convictions at the expense of false acquittals should affect the choice of a “significance level” for deciding whether pieces of glass match in their chemical composition (and hence might have a common origin). Must a criminalist favor the hypothesis that similarities are coincidental over the hypothesis that the fragments have a common origin? The underlying issue applies to many forms of identification evidence, including fingerprints, fibers, paint chips, bullets, and biological fluids. Indeed, arguments over the choice of a significance level arise for statistical evidence of all sorts, from econometrics to epidemiology.⁵