The issue of paternity depends not only on consideration of the genetic evidence (the Combined Paternity Index), but on also consideration of the non-genetic evidence, which includes fertility, access of the tested man to the mother, the issue of other possible fathers, etc. As non-genetic evidence is based on events occurring near the time of conception, it forms the basis of Prior Probability, which is the probability of paternity before testing.
To maintain neutrality, laboratories assume that, before testing, the tested man has a 50/50 chance of being the father (i.e. prior probability = 50%, or 0.5). After testing, however, the probability of paternity changes due to the newly-obtained genetic evidence. If the genetic tests exclude the tested man, then the probability of paternity will drop to 0%. If the DNA tests do not exclude the tested man, then the probability of paternity will increase to more than 99%.
Posted in: Paternity Testing - Basics