Roger Hui wrote: >> John's version relies on ! which is limited to !170. When the mean >> exceeds >> 170, as Mr. Hui suggested, perhaps the function should simply return a >> call >> to the normal function. > > Alternatively, you could recode tpdf to use equation 5 in > http://mathworld.wolfram.com/Studentst-Distribution.html > and compute the ratio (gamma 0.5*1+x)%gamma 0.5*x > not directly but by */vec0%vec1 . > > Separately, another possibility is to use the exact method > up to its limit and apply integration on the remaining tail.
Some of this depends on what accuracy you are shooting for and over what domain. SPSS regards 3 decimal places as adequate for P-values; as a rule of thumb, you can use the normal distribution if the number of degrees of freedom is over 30; and you never need y large (apart from nu=1, all the critical values for alpha=0.005 are less than 10). If the aim is to produce a function accurate over a larger domain, a more patchwork approach may be needed. Best wishes, John ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
