Paternity testing. 2: Likelihood ratio tests.

It is known that the so-called paternity index is also a likelihood ratio statistic for testing that an alleged father is the true father. Unfortunately the likelihood ratio test can sometimes lead to unsatisfactory results because of its dependence on the phenotype combinations of the mother and child. A new, conditional likelihood ratio test, which we call the ancillary test, is proposed in which statistical testing is carried out conditionally on the phenotypes of the mother and alleged father. An exact procedure is available which can be executed with reasonable facility on a microcomputer.