To understand the Combined Paternity Index it is important to know something about the genetic loci analyzed. A paternity DNA test results will invariably make reference to this key terms. Scientists will analyze a number of genetic loci, which are very specific locations on your DNA. For each locus (many labs take 21 such loci), they calculate a specific paternity index. When they have a number for each one of the indices, they combine them together by means of a mathematical formula to get what is known as the combined paternity index. The combined paternity index or CPI, basically tells you the likelihood of one man being the biological father as opposed to any other random man in a given population. You can read more about this and your DNA test results if you wish to get a good grasp of certain other related terms.
The combined paternity index is then expressed as a percentage which is known as the probability of paternity.