Full_Name: Ken Kelley Version: 2.2 OS: Windows XP Submission from: (NULL) (156.56.3.241) It seems that the pf() function when used with noncentral parameters can behave badly at times. I've included some examples below, but what is happening is that with some combinations of df and ncp parameters, regardless of how large the quantile gets, the same probability value is returned. Upon first glance noncentral values greater than 200 may seem large, but they are in some contexts not large at all. The problems with pf() can thus have serious implications (for example, in the context of sample size planning). I noticed that in in 1999 and 2000 issues with large degrees of freedom came about (PR#138), but I couldn't find the present issue reported anywhere. # <Begin example code> X <- seq(10, 600, 10) # Gets stuck at .99135 ############################ round(pf(X, 10, 1000, 225), 5) round(pf(X, 10, 200, 225), 5) round(pf(X, 5, 1000, 225), 5) round(pf(X, 5, 200, 225), 5) round(pf(X, 1, 1000, 225), 5) round(pf(X, 1, 200, 225), 5) # Gets stuck at .97035 ############################ round(pf(X, 10, 1000, 250), 5) round(pf(X, 10, 200, 250), 5) round(pf(X, 5, 1000, 250), 5) round(pf(X, 5, 200, 250), 5) round(pf(X, 1, 1000, 250), 5) round(pf(X, 1, 200, 250), 5) # Gets stuck at .93539 ############################ round(pf(X, 10, 1000, 275), 5) round(pf(X, 10, 200, 275), 5) round(pf(X, 5, 1000, 275), 5) round(pf(X, 5, 200, 275), 5) round(pf(X, 1, 1000, 275), 5) round(pf(X, 1, 200, 275), 5) # <end example code>