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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.
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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
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 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
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
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.