Narrator: This is Science Today. DNA testing is a new tool for catching criminals that's catching on in court. But criminologist William Thompson of the University of California, Irvine says it's not just a matter of matching a suspect's DNA to that of a sample found at the crime scene. There are variables that juries need to be aware of.

Thompson: One variable of course is the possibility of a false positive error, that is, a laboratory error. Another variable that has to be taken in to account is the quality of the match. Sometimes the match is very certain, the quality of the evidence is extremely good. Sometimes the match is only approximate, and that needs to be taken into account.

Narrator: A third variable is when a suspect's DNA is tested against a sample containing DNA from several people.

Thompson: Then the probability that any person at random would match that goes up, because there are multiple ways to match.

Narrator: Thompson believes DNA testing -- and the way it's presented in court -- will grow more sophisticated with time. For Science Today, I'm Steve Tokar.