Tuesday, March 09, 2004

A Problem in Relevance, Conditional Relevance, and Dependent Conditional Probability

A crime is committed. The perpetrator leaves behind a shirt. The issue at trial is identity: Did the defendant commit the crime or was it someone else? At the trial the prosecutor offers to show that twenty different dogs at twenty different times were allowed to sniff the shirt left at the scene of the crime and that each of those dogs separately led the dog handlers to the defendant.

Evidence is also presented at the trial about the ability or inability of each of the 20 dogs to follow a scent. The jurors consider each dog and the evidence about each dog separately. They conclude, in each instance, that each dog more probably than not cannot follow a scent.

Should the jurors have been told in this case to disregard evidence about the tracking behavior of the dogs (that each of the twenty dogs led dog handlers to the defendant) if the jurors conclude that it is more probable than not that each dog cannot follow a scent? Alternative statement of the problem: Should the trial judge refuse to admit the dog-tracking evidence if the trial judge concludes that there is insufficient evidence to permit a reasonable jury to conclude that it is more probable than not that each dog is capable of tracking a scent -- if, that is, the trial judge concludes that a reasonable jury would have to find that it is more probable than not that each dog cannot follow a scent?

See Federal Rules of Evidence 104(b), 401 & 402.

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