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Database Searches. Non-random samples of N individuals Typically individuals convicted of some crime Maryland, people arrested but not convicted. What does the scientific community think happens?.
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Database Searches Non-random samples of N individuals Typically individuals convicted of some crime Maryland, people arrested but not convicted
What does the scientific community think happens? • Balding and Donnelly (1996):”..a search is made through the database with the result that exactly one of the profiles in the database is found to match the crime scene profile.” • NRCII: “..the suspect is initially identified by searching a database to find a DNA profile matching that left at a crime scene.” • Stockmarr (1999): “..exactly one profile in D is found to match eTP” • Evett and Weir (1998): “..the profile from the vaginal swab was searched against the database and his profile was found to match.”
So what’s a match? • Evett and Weir (1998): They suppose that there are two samples, a crime and suspect sample that are typed by DNA techniques. The samples match when, “The two samples are found to be of the same type.”
What really happens? • Target profile and database, candidate, profiles may be compared by two different criteria: high stringency and moderate stringency • High stringency has the usual meaning of a match, that every allele in the target must be seen in the candidate and there can be no extra alleles in the candidate profile
Moderate Stringency • If the target is a mixture with three or more alleles then matching candidates would be any of the possible pairs of samples • Target = 9,11,14 • Moderate matches = 9/9, 9/11, 9/14, 11/11, 11/14,14/14 • This is similar to computing included genotypes in mixtures
Moderate Stringency: one allele • If either the target or the candidate profile has only one allele, then moderate stringency matches are all genotypes with at least one copy of the single allele • Target profile = 12 • Matching candidates = 12/X, where X is any other allele • This criteria is more generous than the typical definitions for mixture inclusions
Implications • Vague protocols for matches by labs creates substantial liability on what constitutes a match. • Example: single source target profile 13/13, would normally not be called a match to a 12/13 candidate • Since labs insist on invoking allelic drop out the possibility of this type of match must always be considered relevant
Statistical Implications • The class of matching genotypes to a 13/13 profile is then 13/X, which is greater than the frequency of the 13 allele but less than twice that frequency • Apparently in California the moderate criteria is turned on in all searches • Even if the search identifies a candidate that matches at high stringency the statistical penalty for the moderate search must be paid (see Venegas)
Are Balding, Donnelly, Evett, Weir, NRCII, and Stockmarr idiots? • NO! However, because almost all forensic labs do not allow access to their software or databases, the scientific community is unaware of the real match criteria • “If scientific evidence is not yet ready for both scientific scrutiny and public re-evaluation by others, it is not yet ready for court.” (NRC I)
Typing Errors • Evett and Weir assume that the chance of not finding a match if the true perpetrator is in the database is zero. • The difference at THO1 is a typing error • FBI Bahamian [2118] 15 /16 17 /20 23 /24 10 /12 30 /32.2 15 /16 11 /13 8 /11 10 /12 9 /12 11 /11 8 /9 9 /11 • [2163] 15 /16 17 /20 23 /24 10 /12 30 /32.2 15 /16 11 /13 8 /11 10 /12 9 /12 11 /11 8 /10 9 /11
Solutions • Any lab following NRCII or likelihood ratios would have to compute random match probabilities adding up all potential matching profiles • Avoid these headaches and follow the NRC I recommendations
