The commonly accepted indicator used for denoting the existence of paternity. It represents the mathematically computed probability that the putative father is the true father of the child, as opposed to any other man of similar racial background. The paternity index, computed using results of various paternity tests following accepted statistical principles for the computation of probability, shall be in accordance with the method of expression accepted by the American Association of Blood Banks.

Summarizes the genetic evidence of a test, and reflects how common a shared allele is in the population. If the PI is close to 1, the allele is fairly common. The larger the PI, the more significant the match. Specifically, the PI is the ratio between the likelihood that the tested man is the source of the paternal allele and the likelihood that a random person is the source.

A paternity index is the numerical indication of the strength of the genetic evidence in a paternity test. The number generated indicates the number of times more likely the alleged father is the biological father of the child than a random man in the population. A PI is calculated for each locus, and its magnitude is dependent on how common or rare the shared allele is between the AF and C.