The Paternity Index (PI) is the genetic odds in favor of an alleged father being the biological father of the child. A PI is calculated for each genetic locus, and represents the likelihood that the alleged father contributed the paternal alleles versus an untested, unrelated, random man in the population. The Combined Paternity Index (CPI), which is calculated by multiplying the individual PIs together, summarizes the overall strength of the genetic evidence.
For example, if the CPI is 100,000, it means that the alleged father is 100,000 times more likely to be the biological father of the child than an untested, unrelated, random man in the population. In other words, the chance that he has matched the child by pure coincidence would be 1 in 100,000. If the alleged father does not possess the paternal alleles of the child at three or more loci, then the PI at each non-matching locus is 0, the CPI is 0, and the alleged father is excluded as the biological father.
Posted in: Paternity Testing - Basics