First, are you talking about rounding error (due to floating point arithmetics) or statistical sampling error?
If you are talking about the latter, I suggest you look it up in a statistics text book. E.g. if x and y are normally distributed, then t = r * sqrt( (n-2)/(1-r**2) ) has a Student t-distribution with n-2 degrees of freedom. And if you don't know how to get the p-value from that, you should not be messing with statistics anyway. -- http://mail.python.org/mailman/listinfo/python-list
