Chapter 4 - National DNA Databases, Strength of Evidence and Error Rates

Abstract

National DNA databases are collections of reference samples from known individuals. A crime stain that matches a reference sample is not itself proof of any offence; rather it is to be regarded as investigative. There has been much debate on the meaning of the strength of evidence of DNA profiles discovered by database searches, and there is a general consensus that it slightly increased as a result of the search. However, these ideas need revision since they have been primarily discussed in relation to an idealized scenario where all but one individual on the database “matches” a crime stain. For complex (partial) DNA profiles, the concept of “match” or “non-match” does not apply in the same way. It is possible that there are several chance matches per database search. Conversely, there may be a failure to match a perpetrator who is actually present on the database, resulting in his wrongful elimination. There is a discussion on the use of Bayesian statistics to calculate strength of evidence in relation to the population size, the error rates, and the non-DNA evidence.