Cold Hit Dna Cases In California
Statistical Significance of a Cold Hit DNA Match
"Forensic DNA analysis is a comparison of a person's genetic structure with crime scene samples to determine whether the person's structure matches that of the crime scene sample such that the person could have donated the sample." (People v. Nelson (2008) 43 Cal.4th 1242, 1257-1258.)
"Once a match is found, the next question is the statistical significance of the match. . . . 'Experts calculate the odds or percentages - usually stated one in some number - that a random person from the relevant population would have a similar match." (Id., at pp. 1258-1259.)
A match may be made by comparing a known suspect's DNA profile to DNA found at the crime scene.
In Nelson our Supreme Court held that, in a cold hit case, the product rule's method of statistical analysis is not subject to the Kelly test 4 and is admissible because it generates relevant evidence. (Id., at pp. 1259-1260, 1266.) Nelson was decided after appellant's trial.
A second method for calculating the statistical significance of a match in a cold hit case " 'was suggested by the National Research Council in 1992.' " (Nelson, supra, 43 Cal.4th at p. 1261.)
This method's approach "would give a result that is reliable, although one that might be unnecessarily conservative." (Ibid.)
In Nelson our Supreme court noted that, in United States v. Jenkins (D.C.Ct.App. 2005) 887 A.2d 1013, 1022, fn. 17, the highest court in the District of Columbia did not address "this approach because it 'is no longer accepted or followed by the relevant scientific community.' " (Nelson, supra, 43 Cal.4th at pp. 1261-1262.)
A third method is the " 'Balding-Donnelly' approach." (Nelson, supra, 43 Cal.4th at p. 1263.) "This method would result in evidence slightly more favorable to the prosecution than would use of the product rule." (Ibid.) It has been "criticized . . . as inherently confusing, difficult to explain to a jury, and possibly misleading." (Ibid.)
The final method has been referred to as the " 'database match probability' " calculation "because it gives the probability of a match from the database." (Nelson, supra, 43 Cal.4th at p. 1262.)
In Nelson our Supreme Court observed that the Jenkins court had "noted that 'the "database match probability" approach . . . more accurately represents the chance of finding a cold hit match' and 'can overcome the "ascertainment bias" of database searches. "Ascertainment bias" is a term used to describe the bias that exists when one searches for something rare in a set database.'" (Id., at p. 1266)
To explain how the database match probability method can overcome the ascertainment bias of database searches, the Nelson court gave the following example: "Assume the product rule calculated a random match odds of one in 1,000,000.
If a single suspect were compared and a match found, the result would be surprising unless the suspect were the actual donor of the evidence. But if a database of 100,000 were searched, the odds - or database match probability - would be about one in 10 that a match would be found even if the actual donor were not in the database.
Thus, a match would be less surprising. If the database had a million profiles, at least one match would be expected even if the actual donor were not in the databank." (Nelson, supra, 43 Cal.4th at p. 1262.)
In Nelson the database contained about 184,000 profiles. Pursuant to the database match probability method, "the odds for Hispanics, the group producing odds most favorable to defendant, would be about one in five followed by 18 zeros." (Nelson, supra, 43 Cal.4th at p. 1262, fn. 1.)
Pursuant to the product rule method, the odds for Hispanics were one in 930 followed by 21 zeros. (Id., at p. 1249.)
In holding that the product rule calculation is admissible in cold hit cases, the Nelson court did not preclude admission of the database match probability calculation:
"The conclusion that statistics derived from the product rule are admissible in a cold hit case does not mean that they are the only statistics that are relevant and admissible.
The database match probability statistic might also be admissible. . . . It is unlikely the database match probability statistic would have been significant to the jury in this case given the size of even that number. But in a different case, if the database were large enough and the odds shorter than those here, the database match probability statistic might also be probative. Nothing we say prohibits its admission." (Nelson, supra, 43 Cal.4th at p. 1267, fn. 3.)