BRATIO

Algorithm 708: Significant digit computation of the incomplete beta function ratios. An algorithm is given for evaluating the incomplete beta function ratio Ix(a,b) and its complement 1 - Ix(a,b). A new continued fraction and a new asymptotic series are used with classical results. A transportable Fortran subroutine based on this algorithm is currently in use. It is accurate to 14 significant digits when precision is not restricted by inherent error. (Source: http://dl.acm.org/)

This software is also peer reviewed by journal TOMS.