What Does A Match Mean?
DNA identification is based on probabilities. Consider the case of just three CODIS sites. The probability that someone would match a random DNA sample at any one site is roughly one in ten (1/10). So the probability that someone would match at three sites would be about one in a thousand:
1/10 x 1/10 x 1/10 = 1/1000
Applying this probability equation to all 13 CODIS sites would mean that the chances of matching a random DNA sample are about one in ten trillion:
1/10 x 1/10 x 1/10 x 1/10 x 1/10 x 1/10 x 1/10 x 1/10 x 1/10 x 1/10 x 1/10 x 1/10 x 1/10 x = 1/10,000,000,000,000
Actual probabilities vary, depending on several factors. But the probability of two different people matching at all 13 CODIS sites is virtually zero.

Visit the Marian Koshland Science Museum to learn more.
See how DNA fingerprints are made and hear how the use of DNA evidence has affected the criminal justice system. 
