SUBJECTIVE INTERPRETATION, LABORATORY ERROR AND THE VALUE OF FORENSICDNA EVIDENCE - 3 CASE-STUDIES

This article discusses two factors that may profoundly affect the valu e of DNA evidence for proving that two samples have a common source: u ncertainty about the interpretation of test results and the possibilit y of laboratory error. Three case studies are presented to illustrate the importance of the analyst's subjective judgments in interpreting s ome RFLP-based forensic DNA tests. In each case, the likelihood ratio describing the value of DNA evidence is shown to be dramatically reduc ed by uncertainty about the scoring of bands and the possibility of la boratory error. The article concludes that statistical estimates of th e frequency of matching genotypes can be a misleading index of the val ue of DNA evidence, and that more adequate indices are needed. It also argues that forensic laboratories should comply with the National Res earch Council's recommendation that forensic test results be scored in a blind or objective manner.