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In the absence of a local database, a national white database is used. Graphs (examples are Figures 5.3 and 5.4, p 150 and 152) show that the individual values that are possibly incorrectly estimated lie within 10-fold above and below the ''correct" value. We conclude that it is reasonable to regard calculated multilocus match probabilities as accurate within a factor of 10 either way. This is true for various subsets within the white, black, Hispanic, and east Asian populations. However, if the database from the wrong racial group is used, the error may be larger (Figure 5.5, p 153). That argues for the use of the correct racial database if that can be ascertained; otherwise, calculations should be made for all relevant racial groups, i.e., those to which possible suspects belong. The databases should be large enough to have some statistical accuracy (at least a few hundred persons), and alleles represented fewer than five times should be rebinned (grouped so that no bin has fewer than five).

Additional information comes from comparison of profiles within the databases. An early study used FBI and Lifecodes data for blacks, whites, Southeast Hispanics, and Southwest Hispanics. Among 7,628,360 pairs of profiles from within those databases, no four- or five-locus matching profiles were found, and only one three-locus match was seen. A newer and more extensive analysis, compiling data from numerous TWGDAM sources, summarized a large number of profiles from white, black, and Hispanic databases. Of 58 million pairwise comparisons within racial groups, only two possible four-locus matches were found, and none were found for five or six loci.

We conclude that, when several loci are used, the probability of a coincidental match is very small and that properly calculated match probabilities are correct within a factor of about 10 either way. If the calculated probability of a random match between the suspect and evidence DNA is 1/(100 million), we can say with confidence that the correct value is very likely between l/(10 million) and 1/(l billion).

PCR-Based Tests

As already mentioned, PCR-based tests have a number of advantages. They include the ability to identify individual alleles, as well as simplicity and quick turn-around. But there are disadvantages. Most of the loci used have a small number of alleles, so that many more loci are required for the same statistical power as provided by a few VNTRs. STRs are also based on repeating units, have a high mutation rate (although not as high as some VNTRs), have a fairly large number of alleles, and are usually capable of unique allelic identification. With 12 STR loci, there is discriminatory power comparable to that of four or five VNTRs, and comparisons between geographical and racial groups show similarities and differences comparable to those of VNTRs.

The quantity , which we use as a measure of population substructure, is determined largely by the population history rather than by the frequency of the