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June 22, 2008

Rounding Up the Usual Suspects III: People v. Nelson

On April 5, 2008, I mentioned People v. Nelson, 48 Cal.Rptr.3d 399 (Ct. App. 3 Dist. 2006), rev. granted, 147 P.3d 1011 (Cal. 2006), as a leading case on the admissibility of the various probabilities associated with cold hits in DNA databases. Last week, the California Supreme Court affirmed.

The case arose from the rape and murder of a nineteen-year-old college student in 1976. Dennis Nelson was a suspect, but the evidence was inconclusive, and the case grew cold. Later, Nelson was convicted of a different rape. His DNA profile was entered into the state convicted-offender databank. In 2001, investigators discovered that this profile matched those derived from stains from the 1976 rape. At that point, there were 184,000 profiles in the database. According to the state, the match would occur “at random among unrelated individuals in about one in 950 sextillion African-Americans, one in 130 septillion Caucasians, and one in 930 sextillion Hispanics.” As the court adds, “[t]here are 21 zeros in a sextillion and 24 zeros in a septillion.”

Nelson moved to dismiss the resulting charges on the ground that the delay between the 1976 crime and the charges filed in 2002 deprived him of his right to a speedy trial. The superior court denied the motion. At trial, Nelson conceded that he had intercourse with the victim but claimed that it was consensual -- somebody else must have murdered her and left her body in the mud. That did not work either. The jury convicted Nelson of first degree murder, and the Court of Appeal affirmed.

The California Supreme Court reviewed two claims. First, with respect to the speedy-trial issue, it held that the 26-year delay between the offense and the prosecution caused only slight prejudice and was justified.

Second, the court considered whether the vanishingly small random-match probabilities should have been admitted. The court correctly held that inasmuch as the procedure underlying this calculation was generally accepted and uncontested, the only real issue was the relevance of a random-match probability in a database-trawl case.

At this point, however, the opinion unravels. It contains but a single, short paragraph to show why the statistic is relevant:

        In a non-cold-hit case, we said that “[i]t is relevant for the jury to know that most persons of at least major portions of the general population could not have left the evidence samples.” (People v. Wilson, supra, 38 Cal.4th at p. 1245.) We agree with other courts that have considered the question (the Court of Appeal in this case; People v. Johnson, supra, 139 Cal.App.4th 1135; and Jenkins, supra, 887 A.2d 1013) that this remains true even when the suspect is first located through a database search. The database match probability ascertains the probability of a match from a given database. “But the database is not on trial. Only the defendant is.” (Modern Scientific Evidence, supra, § 32:11, pp. 118-119.) Thus, the question of how probable it is that the defendant, not the database, is the source of the crime scene DNA remains relevant. (Id. at p. 119.) The rarity statistic addresses this question.

As the co-author of the text of the treatise being quoted, I fear that these words are inconsistent with the portion of the court's opinion (in note 3) suggesting that the database-match probability also is relevant. If the issue is simply “how probable it is that the defendant, not the database, is the source of the crime scene DNA,” then the database-match probability is irrelevant. Unlike the “rarity statistic,” it does not figure into the probability that the named defendant is the source. The formulas are given and explained in a forthcoming article, Rounding Up the Usual Suspects: A Logical and Legal Analysis of DNA Trawling Cases. The Nelson court's theory that a variety of statistics are admissible in a database-trawl case does not withstand analysis. Or, if it does, it will take more analysis than this court has provided to explain why. The opportunity for such clarification may well arise, as there will some cases in which defense counsel will be interested in introducing the database-match probability, which can be orders of magnitude larger than the random-match probability.

In this particular case, however, the demand for an adjustment to the random-match probability is much ado about nothing. So what if the probability is 10–19 rather than 10–24? Having rejected the defense argument about general-acceptance, the court could simply have observed that the choice of a statistic could not have affected the outcome of the case. The court realized this, but it endorsed the 10–24 figure anyway.

-- DHK


People v. Nelson, No. S147051 (Cal. June 16, 2008), slip opinion available at http://www.courtinfo.ca.gov/opinions/documents/S147051.pdf

Dolan, Maura and Jason Felch. 2008. "California Supreme Court Ruling Allows 'Rarity' Statistic in DNA Cases." Los Angeles Times: June 17 available at  http://www.latimes.com/news/science/la-me-dna17-2008jun17,0,3313471.story

Kaye, David H. 2009. "Rounding Up the Usual Suspects: A Logical and Legal Analysis of DNA Trawling Cases". North Carolina Law Review: in press. Prepublication draft available at SSRN: http://ssrn.com/abstract=1134205

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