# Cold Hit Statistics NRC 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.

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Cold Hit Statistics NRC 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

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

Advantages and Disadvantages Standard statistics, avoid controversy Existence of databases doesnt have to be mentioned Easy to accommodate search to any subset of the CODIS 13 loci Lose information contained in database search match

Is there really a disadvantage? Prob{suspect matches perp profile|suspect profile,perp 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, PNAS 92:11741

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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