NCJ Number
225884
Journal
Journal of Forensic Sciences Volume: 54 Issue: 1 Dated: January 2009 Pages: 22-27
Date Published
January 2009
Length
6 pages
Annotation
This analysis of hypothetical cases in which the DNA profile of a crime-scene sample is found to match a known profile in a DNA database (“cold hit”) focuses on the average probability that someone not in the database but in the population also has the same profile as the crime-scene sample and the average probability of an erroneous attribution, i.e., that the crime was actually committed by someone in the population whose profile is not in the database.
Abstract
This study shows that the average probability that there is another person in the population whose profile matches the crime-scene sample but who is not in the database is approximately 2(N - d)pA. N is the number of individuals in the population; d is the number of profiles in the database, and pA is the average match probably (AMP) for the population. The AMP is estimated by computing the average of the probabilities that two individuals in the population have the same profile. In addition, the study shows that if a priori each individual in the population is equally likely to have left the crime-scene sample, then the average probability that the database search attributes the crime-scene sample to a wrong person is (N - d)pA. The chance of an erroneous attribution is thus very small, even under the conservative assumptions made in the study. Although study results apparently support the existing practice of using “cold hits” as valid indications of the presence of the suspect (persons whose DNA in the database matched the crime-scene sample) at the crime scene, it also calls attention to the critical role of the assumption of independence across loci in justifying the use of cold hits to obtain convictions. 40 references and appended derivation of average probabilities