systems in common forensic use, in which a single sample might yield somewhat different allele sizes on repeat measurements.1 It is easy to determine whether two samples match in the former case (assuming that the patterns have been correctly identified), but the latter case requires a match criterion—i.e., an objective and quantitative rule for deciding whether two samples match. For example, a match criterion for VNTR systems might declare a match between two samples if the restriction-fragment sizes lie within 3% of one another.

The match criterion must be based on the actual variability in measurement observed in appropriate test experiments conducted in each testing laboratory. The criterion must be objective, precise, and uniformly applied. If two samples lie outside the matching rule, they must be declared to be either ''inconclusive" or a "nonmatch." Considerable controversy arose in early cases over the use of subjective matching rules (e.g., comparison by eye) and the failure to adhere to a stated matching rule.

Identification of Potential Artifacts

All laboratory procedures are subject to potential artifacts, which can lead to incorrect interpretation if not recognized. Accordingly, each DNA typing method must be rigorously characterized with respect to the types of possible artifacts, the conditions under which they are likely to occur, the scientific controls for detecting their occurrence, and the steps to be taken when they occur, which can range from reinterpreting results to correcting for the presence of artifacts, repeating some portion of the experiment, or deciding that samples can be reliably used.

Regardless of the particular DNA typing method, artifacts can alter a pattern in three ways: Pattern A can be transformed into Pattern B, Pattern A can be transformed into Pattern A + B; and Pattern A + B can be transformed into Pattern B. It is important to identify the circumstances under which each transformation can occur, because only then can controls and corrections be devised. For example, RFLP analysis is subject to such artifacts as band shifting, in which DNA samples migrate at different speeds and yield shifted patterns (A→B), and incomplete digestion, in which the failure of a restriction enzyme to cleave at all restriction sites results in additional bands (A→A + B).

Some potential problems can be identified on the basis of the chemistry of DNA and the mechanism of detection in the genetic-typing system. Anticipation of potential sources of DNA typing error allows systematic empirical investigation to determine whether a problem exists in practice. If so, the range of conditions in which an assay is subject to artifact must be characterized. In either case, the results of testing for artifacts should be documented. Empirical testing is necessary, whether one is considering a new method, a new locus, a new set of reagents (probe or enzyme) for a

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