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uncontroversial, quantitative estimates of how rare the identifying characteristics are within particular groups and subgroups. Whether many other forms of identification-evidence could survive comparable demands is doubtful.2 Jurists and legal scholars have debated whether DNA evidence warrants this special treatment.3 We take no sides in such legal debates, but we do emphasize that the two issuesthe scientific acceptability of the laboratory method for comparing samples and the idea that the characteristics studied in the laboratory are probative of identityare distinct. Consequently, this chapter describes the implications of our conclusions about the state of scientific knowledge both for testimony about the extent to which DNA samples match and for testimony about the probabilities of such matches.
Legal Standards and Procedures
Whether scientific evidence is admissible in criminal cases depends on whether the evidence tends to prove or disprove a fact that, under the applicable law, might matter to the outcome of the case; whether the expert presenting the evidence is qualified; whether the information is derived from scientifically acceptable procedures; and whether the potential for unfair prejudice or time-consumption substantially outweighs the probative value of the information. We discuss those general principles and then consider their application to DNA evidence. We also describe pretrial and trial procedures that might help courts to reach decisions on admissibility and to improve the quality and use of the scientific evidence at trial. We begin with the intertwined procedural issues that arise in connection with a defendant's request for discovery, retesting, or expert assistance.
The Defendant's Right to Discovery
The 1992 National Research Council (NRC) report stated that ''all data and laboratory records generated by analysis of DNA samples should be made freely available to all parties," and it explained that "all relevant information . . . can include original materials, data sheets, software protocols, and information about unpublished databanks" (NRC 1992, p 150, 148). Certainly, there are no strictly scientific justifications for withholding information in the discovery process, and in Chapter 3 we discussed the importance of full, written documentation of