Thursday, November 06, 2003

Crunching the DNA Numbers

DNA evidence seems a more humdrum affair in courts outside California. The New York Times is running this story on Wednesday's seemingly uneventful testimony from an FBI forensic expert in the DC sniper trial. The expert opined that genetic material found on a Bushmaster rifle allegedly used in the shootings almost certainly belonged to defendant John Muhammed. The chances that the DNA belonged to someone else, the expert said, were less than one in 46 billion.

It is common to hear such probabilistic claims in DNA testimony. But where do the experts get these numbers? How are the probabilities computed? A fairly lucid primer on DNA identification, with some discussion of the number crunching, can be found at
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.