Algorithm and series expansion for the F distribution
William E. Davis, Hatem M. Khalil · ACM SIGNUM Newsletter · 1972
POFF computes the probability of F values no greater than some assigned value. The method of evaluation is based on expressing the probability function in a simple algebraic form which is evaluated as a nested polynomial. In this respect it differs from Algorithm 322 [1] which computes the same function by recurrences. A comparison of their efficiency shows that POFF is faster but the core requirements and the accuracy of the results are identical.