A comparison of the fixed bin method with the floating bin and direct count methods: effect of VNTR profile frequency estimation and reference population.

When the results of a forensic comparison of highly polymorphic variable number tandem repeat (VNTR) loci fail to exclude a suspect as a possible contributor of biological evidence, it is desirable to convey to the trier of fact the significance of the match. Furthermore, in a forensic context, it is desirable that the estimated frequency of occurrence be conservative, that is, that any uncertainty in the estimate will favor the accused. Using an empirical approach with a data base of 2046 individuals belonging to one of four population groups, this study examined the effect of the method used to estimate frequency of occurrence of a VNTR profile from a reference data base, and the consequences of using a data base that may not represent the circumstances of the crime. The fixed bin method was at least as conservative as the floating bin and genotype counting (direct counting) methods. Secondly, for forensic purposes, profile frequency estimates from different reference populations do not deviate greatly. VNTR profiles are rare in any of the data bases.