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Reiteration of the Statistical Basis of DNA Source Attribution Determinations in View of the Attorney General's Directive on “Reasonable Scientific Certainty” Statements

Sir, With the advent of short tandem repeat (STR) typing for forensic DNA applications in the 1990s, the National Research Council (NRC) of the National Academy of Sciences in 1996 and the Federal DNA Advisory Board (DAB) which was established under the DNA Identification Act of 1994 recognized that the inverse of the rarity of a multilocus DNA profile can exceed the world population many fold and, consequently, the chance of two individuals other than identical twins having the same profile is remote. Both the NRC and DAB advocated the use of source attribution to qualify a rare match probability derived from an evidentiary DNA profile (1,2). Given this foundation, the FBI Laboratory has reported source attribution of DNA profiles since 1997 using the approach described by Budowle et al. (3), which entails a mathematical formula and specified “objective criteria [that] can be used to report that with reasonable scientific certainty a particular individual is the source of an evidentiary sample”. In a September 6, 2016, Department of Justice memorandum (4), the U.S. Attorney General responded to recommendations from the National Commission on Forensic Science (5) and directed “Department forensic laboratories [to] review their policies and procedures to ensure that forensic examiners are not using the expressions ‘reasonable scientific certainty’ or ‘reasonable [forensic discipline] certainty’ in their reports or testimony.” Referencing the phrase “reasonable degree of scientific certainty” based on conventions that differ substantially from what is meant in the context of source attribution of a DNA profile (3), the Commission maintained that “there is no common definition within science disciplines as to what threshold establishes ‘reasonable’ certainty” and that the phrase is not required by courts of law. The Commission asserted that the phrase has “no scientific meaning” and “may mislead fact finders about the level of objectivity involved in the analysis” if presented in court. This communication serves to reiterate the statistical criteria published in 2000 (3) upon which the FBI Laboratory has reported and provided testimony attributing “to a reasonable degree of scientific certainty” an individual as the source of a DNA profile. We distinguish the criteria explicated by Budowle et al. (3) from any indeterminate use of the phrase in other disciplines and legal settings. Indeed, attribution of source may be reported without use of the “reasonable degree of scientific certainty” phrase. Herein, all assumptions, calculations, and degree of confidence are discussed so that fact finders and other stakeholders can be informed on what the use of the phrase “to a reasonable degree of scientific certainty” means in the context of the case specific DNA profile. If upon examination of the DNA typing results, a person of interest cannot be excluded as a possible contributor to the evidence, in accordance with Standard 11.2.6 of the Quality Assurance Standards for Forensic DNA Testing Laboratories, a match probability [e.g., random match probability (RMP), combined probability of inclusion, likelihood ratio] must be reported as a quantitative statement of the weight of the evidence (6). The DAB advised that “For forensic applications, it is important that the statistical conclusions be conveyed meaningfully. . . Simplified approaches are appropriate, as long as the analysis is conservative or does not provide false inferences.” (2). An inherent certainty rather than a possibility may be perceived by the infinitesimally small match probability resulting from, for example, a thirteen-locus STR profile, which on average is less than 1 in a trillion. Indeed, it can be difficult to conceptualize that anyone other than a defendant could have contributed DNA to the evidence if the inverse of the match probability exceeds the size of the human population as a whole, especially by several orders of magnitude. The NRC opined,