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B. DNA Testing: Fraught With Peril

Narrator: This is Science Today. More suspects are being convicted -- or freed -- based on DNA testing. Criminologist William Thompson of the University of California, Irvine says the problem is that juries often don't understand exactly what the test means.

Thomson: When the evidence is used to incriminate somebody, basically the laboratory is saying that they find a match or a similarity between the genetic characteristics of two samples. And because the genetic characteristics of samples is not something that people in their everyday experience know a lot about, you have to say something to the jury to explain to them the meaning of a match.

Narrator: Thompson says statistics like "one in a billion match" are meaningless by themselves. DNA samples are compared at only a few spots along the sample -- and there's plenty of room for error.

Thompson: And the value of a match across several sites for incriminating somebody varies dramatically depending on how rare the matching characteristics are and how certain we are that what's being called a match is really a match.

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