Academic journal article Forensic Science Communications

Statistical Weight of a DNA Match in Cold-Hit Cases

Academic journal article Forensic Science Communications

Statistical Weight of a DNA Match in Cold-Hit Cases

Article excerpt


On June 6, 2008, the Supreme Court of California ruled that the "Random Match Probability" (RMP) is a relevant statistic for assessing the statistical strength of DNA evidence in a cold-hit case (see People v. Nelson 2008). The ruling also states (see footnote 3 of the decision) that other alternative statistics (such as the "Database Match Probability," DMP) also may be admissible and, more intriguingly, hinted as though RMP (and/or DMP) equates information as to "how likely it is that someone other than the defendant is the source of the crime scene evidence" (People v. Nelson 2008, pp. 30-31).

This decision is not the only discussion on the subject of relevant statistics for assessing the significance of a DNA match in cold-hit cases (see, e.g., Felch and Dolan 2008; Kaye 2008; People v. Johnson 2006; Storvik and Egeland 2007 and its cited references; and United States v. Jenkins 2005). Indeed, in some of these cases, the defense argued for inadmissibility of the entire DNA evidence on the grounds of a "raging debate" over the statistics for DNA evidence in cold-hit cases (see, e.g., State of Arizona v. Luong 2008 and United States v. Jenkins 2005). Although most courts have rejected such arguments, our commentary is prompted by a need to clearly distinguish the different concepts, as well as to clarify scenarios involving cold-hit cases. Confusion arises because often the interpretations of the different statistics are not clearly stated, and hence it is implied that a jury would be misled by a decision based on the RMP and would reverse itself if the court had an opportunity to hear about DMP (see, e.g., Felch and Dolan 2008).

In this commentary we first describe how (if at all) a cold-hit case may differ from a weight-of-evidence point of view in other cases in which DNA evidence is presented. Subsequently, we describe the different approaches for evaluating the strength of a DNA match in cold-hit cases, with the relevant questions that they answer, to establish that different solutions to different questions do not necessarily constitute a controversy. With these differences clearly defined, we argue that there is no disagreement in the literature, or in court testimonies, that the concept or the numerical value of a random-match probability is invalidated because a cold-hit identification of a suspect occurred and that no other number should be advocated to replace the RMP to evaluate its original (and only) interpretation. In fact, we proffer that DMP and RMP actually provide the same strength of a DNA match in cold-hit cases, if the frequentist's interpretation of DMP is correctly spelled out.

Finally, with some brief statements about the rationale of establishing the offender database (the Combined DNA Index System, CODIS) in the United States and in other countries, we surmise that because such systems of suspect identification are becoming more and more successful with the enrichment of such databases, the attempt to modify RMP using database size as additional information without any mention of the RMP value would produce an inconsistent, if not erroneous, statistic, completely unrepresentative of the strength of the DNA evidence.

Probable Cause Versus Cold Hit

Crime investigations generally start with one or more suspects' being identified based on leads obtained from informants or personal identification clues (not related to DNA profiles) left at the crime scene. Such cases are described as "probable cause" cases because the suspect is implicated in the crime on the basis of his or her association with the crime scene based on such leads taken alone or in combination with one another. If the crime scene provides any biological sample that may be from the perpetrator (and, in some cases, the victim), its DNA profile is compared with that of the suspect, and a resultant match constitutes DNA evidence in such a case. Because DNA data confirm the probable-cause scenario, these cases are also referred to as "confirmatory cases. …

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