ok i am trying to compute the sample standard deviation and i am comparing to excels STDEV function (which divides by N - 1 giving the samples standard deviation) my values aren't masssivly different from those given by excel but enough to be worrying i believe the algorithm is right i'm just supprised if a precision difference could cauze the differences i'm seeing i'll post the data in a sec probably getting round the broken ppm is the answer there are so many other things i need to install modules for anyway but i got used to writing my own bits for portability around this crazy network