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DNA - A Prosecutor’s Practice Notebook Inventory

Defense Logic

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The defense logic states that by comparing the suspect's DNA profile to many thousands or millions in a database, the statistical probability of finding a match is much greater than a one-on-one comparison. While this claim is statistically correct, the answer is that the "cold hit" is only used as probable cause to obtain a reference sample from the suspect. Once a reference sample is typed, it is the profile from that sample which is compared to the evidence sample. Thus, the probability calculation performed is in fact a one-on-one comparison of the suspect's reference sample. [DNA Database Searches and the Legal Consumption of Scientific Evidence, 97 Mich. L. Rev. 931, 1999, and; Statistical and Population Genetics Issues Affecting the Evaluation of the Frequency of Occurrence of DNA Profiles Calculated From Pertinent Population Databases, Forensic Science Communications, July 2000, Vol. 2 #3.]

Another reason the defense argument is faulty is that the question the jury is entitled to be informed on is the same in a cold hit case as in any other case: how rare or common is the DNA profile within the population? This is a question of legal relevancy and not a question of proper scientific procedure or methodology.

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