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E. When Is DNA Testing Valid?

Narrator: When is DNA testing valid? This is Science Today. More and more criminal cases hinge on whether a suspect's DNA matches a sample found at the scene of the crime. Criminologist William Thompson of the University of California, Irvine says DNA testing can be a powerful tool, but it can also be misused by the prosecution.

Thompson: What troubles me is the misleading way that DNA evidence is sometimes presented to juries. Juries hear numbers like one in a billion or one in a trillion, they're led to believe that that is the number they should consider when deciding how much weight to give to the evidence, and I think there's every indication that they give it a great deal of weight unless the defense does a good job attacking it.

Narrator: Thompson says defense attorneys should be more aware of when to question DNA test results. Sometimes a match might not be a match. Sometimes a sample might have been contaminated.

Thompson: None of these things prove that the test is wrong, but they're red flags, if you will, that to my way of thinking aren't heeded sufficiently.

Narrator: For Science Today, I'm Steve Tokar.