The Evaluation of Forensic DNA Evidence (1996) / Chapter Skim
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Executive Summary
Executive Summary
Pages 1-8
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In 1992, a broad-ranging report released by the National Research Council attempted to explain the basics of the relevant science and technology, to offer suggestions for improving forensic DNA testing and its use in law enforcement, and to quiet the controversy that had followed the introduction of DNA profiling in court. Yet, the report did not eliminate all controversy.
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From page 2... ...
This report describes both the science behind DNA profiling and the data on the frequency of profiles in human populations, and it recommends procedures for providing various statistics that may be useful in the courtroom. The procedures are based on population genetics and statistics, and they render the ceiling principle and the interim ceiling principle unnecessary.
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From page 3... ...
It concludes that the abundance of data in different ethnic groups within the major races and the methods outlined in Chapters 4 and 5 imply that the 1992 report's suggested ceiling principle and interim ceiling principle are unnecessary. In addition, it makes recommendations to help assure the accuracy of estimates for what are known as VNTR profiles and to handle the special situation in which the suspect was identified as a result of a search through a database of DNA profiles of known offenders.
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From page 4... ...
An unrealistically large number of proficiency tests would be needed to estimate accurately even an historical error rate. For such reasons, proficiency test results should not be combined with the estimated frequency of an incriminating profile to yield the probability that a laboratory would report that DNA from a person selected at random contains the incriminating profile.
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From page 5... ...
In most cases, there is no special reason to think that the source of the evidence DNA is a member of a particular ethnic subgroup within a broad racial category, and the product rule is adequate for estimating the frequency of DNA profiles. For example, if DNA is recovered from semen in a case in which a woman hitchhiker on an interstate highway has been raped by a white man, the product rule with the 2p rule can be used with VNTR data from a sample of whites to estimate the frequency of the profile among white males.2 If the race of the rapist were in doubt, the product rule could still be used and the results given for data on whites, blacks, Hispanics, and east Asians.
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From page 6... ...
This recommendation deals with the case in which the person who is the source of the evidence DNA is known to belong to a particular subgroup of a racial category but there are no DNA data on either the subgroup or the population to which the subgroup belongs. It would apply, for example, if a person on an isolated Indian reservation in the Southwest, had been assaulted by a member of the tribe, and there were no data on DNA profiles of the tribe.
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From page 7... ...
Because the lengths of VNTRs cannot be measured exactly, an uncertainty window surrounds each measured VNTR, and two VNTRs are said to match when their uncertainty windows overlap. To calculate the frequency of matching VNTR profiles, one must find the proportion of VNTRs that fall within a match window around each VNTR in the incriminating profile.
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From page 8... ...
The conceivable alternatives include statements of the posterior probability that the defendant is the source of the evidence DNA, qualitative characterizations of this probability, computations of the likelihood ratio for the hypothesis that the defendant is the source, qualitative statements of this measure of the strength of the evidence, the currently dominant estimates of profile frequencies or randommatch probabilities, and unadorned reports of a match. Courts or legislatures must decide which of these alternatives best meets the needs of the criminal justice system.
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