Saturday, February 17, 2007

Statistical DNA Testimony Upheld by D.C. Court of Appeals

The Court of Appeals for the District of Columbia (a Frye jurisdiction) has upheld testimony on the statistical odds of a DNA match. From the opinion:
The issues in this appeal ... arise from evidence of a DNA match which the FBI found after examining semen removed from panties K.W. had worn on the day of the assault and comparing it to the known DNA profile of appellant. Dr. Frank Samuel Baechtel, a DNA expert from the FBI forensic laboratory, testified that appellant's DNA profile and K.W.'s profile were both consistent with DNA found in the recovered semen. He further opined, using the FBI's highly conservative estimate for cases involving so-called mixed samples, that the statistical chance of finding a person at random who could have been a contributor to the DNA on the panties was no greater than 1 in 410,000 among four major population groups in the United States.

On appeal, appellant makes a variety of challenges to the admissibility of Dr. Baechtel's opinion, arguing particularly (a) that the expert could not properly offer a match-probability statistic without also providing the jury with an "error rate," that is, a "false positive probability to represent the chance of a false match caused by laboratory error"; and (b) that the FBI's method of interpreting mixed samples, which contain DNA of two or more persons, "relies on unwarranted assumptions about the number of contributors and their individual DNA profiles." We hold that, while appellant could properly explore these matters on cross-examination of Dr. Baechtel or through expert testimony of his own, in order to attack the weight of the DNA evidence, they did not render inadmissible Dr. Baechtel's opinion either that appellant could not be excluded as a contributor to the semen found on K.W.'s clothing or as to the probability of a random match.
See Roberts v. United States, No. 03-CF-853 (D.C. Feb. 15, 2007).



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Fed. R. Evid. 702: If scientific, technical, or other specialized knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue, a witness qualified as an expert by knowledge, skill, experience, training, or education, may testify thereto in the form of an opinion or otherwise, if (1) the testimony is based upon sufficient facts or data, (2) the testimony is the product of reliable principles and methods, and (3) the witness has applied the principles and methods reliably to the facts of the case.