Narrator: There's more to DNA testing than meets the eye. This is Science Today. More criminals are being sent to jail based on evidence that their DNA matches a sample found at the crime scene. But criminologist William Thompson of the University of California, Irvine says that in order to be accurate and avoid mistakes, a DNA sample should be split in two and tested several times at two separate labs.

Thompson: But it doesn't happen very often. Most of the cases I see involve a single sample tested by one lab, one time, without replication, without any parallel testing, without any other samples being tested to check for consistency and so on. And if you see that, and if that single test has something problematic about it, that's an issue of concern.
Narrator: Thompson thinks most lawyers ignore the variables involved in DNA testing and oversimplify it instead.

Thompson: It tends to be presented as a black and white thing. The prosecutors say it's fabulous, the defense lawyers say, oh it's all bunk, and of course the truth is -- the truth may well be in between.

Narrator: For Science Today, I'm Steve Tokar