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Understanding Cold Hit Statistics in Forensic DNA Analysis

This article discusses the methodology of cold hit statistics in forensic DNA analysis, specifically how searches are conducted using a subset of available loci. A second set of genetic markers confirms matches, and statistics are calculated using the standard product rule. It details the example of the FBI's use of loci and their respective match probabilities, along with the advantages and disadvantages of this approach. The article also includes references for further reading on DNA technology in forensic science.

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Understanding Cold Hit Statistics in Forensic DNA Analysis

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  1. Cold Hit StatisticsNRC I How does it work? Searches are done with a subset of available loci. A second set of different genetic markers are used to confirm the match. Statistics are only done with the second set of loci. The statistics are the standard product rule

  2. Example: FBI African American • Use six loci for search: D3S1358, vWA, FGA, D8S1179, D21S11, and D18S51 • The average RMP is 1 in 62,000,000 • The chance of a coincidental match in a 3 million person database is about 5% • Confirm the match at D5S818, D13S317, D7S820, CSF1PO, TP0X, TH01, D16S539 • The average match probability for these 7 loci is 1 in 15 million • With the 2 Identifilier loci D2S1338 and D19S433 this average increases to 1 in 16 billion

  3. Advantages and Disadvantages • Standard statistics, avoid controversy • Existence of databases doesn’t have to be mentioned • Easy to accommodate search to any subset of the CODIS 13 loci • Lose information contained in database search match

  4. Is there really a disadvantage? Prob{suspect matches perp profile|suspect profile,perp¹ suspect} = Prob{}=RMP´[1-Prob(false negative)] +(1-RMP)´[Prob(false positive)] Usual assumption: Prob(false negative) = Prob(false positive) = 0, thus Prob{} = RMP However if RMP<<Prob(false positive), then Prob{} @ Prob(false positive) See Balding and Donnelly, 1995, PNAS92:11741

  5. References • Nation Research Council, Committee on DNA Technoogy in Forensic Science. 1992. DNA technology in forensic science. • Morton, N.E. 1997. The forensic DNA endgame. Jurimetrics 37:477-494.

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