Scientific Evidence (law) - Interpreting Witness

Interpreting Witness

Interpreting (Evaluating) Witness: Sometimes not needed 1) when test result is self-explanatory or pass-fail, or 2) when there is a statutory presumption obviating the need (e.g. drunk driving statutes and a test showing raised blood alcohol levels). Otherwise, this witness needed to complete the foundation. Syllogistic in nature: 1) states the interpretive standard (Rule or Major Premise), applies the standard to the test result (minor premise) and derives a conclusion.

• Qualifications: a hybrid with both academic and experiential qualifications.
• Will base finding on the Reporting witness. Ideal if present when Reporter conducted test, but may be permitted in some jurisdictions.
• Some jurisdictions won't accept opinion unless it is a "reasonable scientific opinion." Otherwise, need to consider if the witness can couch the opinion in terms of statistical probability.

For example, in the casebook case of People v. Collins, 438 P.2d 33 (Cal. 1968), an elderly lady was knocked down and robbed by a blond who escaped in yellow car with bearded black man. Defendants met that rough description but could not be conclusively identified. Prosecutor used a mathematics professor to discuss the probability that this couple could be the guilty party. Lower court overruled Defendants' objection. Court held that 1) there was no foundational establishment of the underlying probabilities and 2) the fact that the Defendants' fit a probability model was irrelevant because it doesn't prove they did it.