Full_Name: Jerry W. Lewis Version: 2.10.1 OS: Windows XP Professional Submission from: (NULL) (166.186.168.21) Since pchisq(x,df,ncp,lower.tail,TRUE) is calculated as log(pchisq(x,df,ncp,lower.tail)) it looses accuracy when pchisq(x,df,ncp,lower.tail) is near 1. Accuracy can be maintained in that case by replacing the existing calculation with log1p(-pchisq(x,df,ncp,!lower.tail))