For forensic purposes, the frequency of an allele in a laboratory's databank should be calculated by counting the number of alleles that would be regarded as a match with the laboratory's forensic matching rule, which should be based on the empirical reproducibility of the system. This matching rule must account for both the quantitative reproducibility of forensic measurements in the testing laboratory and the quantitative reproducibility of the population measurements in the laboratory that generated the databank. In addition, the matching rule should reflect that one is making inter-gel comparisons, which are typically less precise than intra-gel comparisons.
The above approach is sometimes referred to as "floating bins," in that one counts the alleles that fall into a "bin" centered on the allele of interest. Most forensic laboratories in this country use the slightly different approach of "fixed bins": One first aggregates alleles into a predetermined set of bins. Given an allele in a forensic case, one must then compute its frequency by adding the frequencies of all the bins that contain any alleles that fall within the window specified by the laboratory's forensic matching rule. (All bin frequencies must be added; it is not enough to take the largest of the bin frequencies.) This fixed-bin approach is acceptable and might be more convenient in some settings, because examiners need only consult a short table of bin frequencies, rather than search an entire databank.