Dear Rick Wicklin Sir, I'am pursuing Ph.D. in Agricultural Statistics from Indian Agricultural research Institute, New Delhi, India. I've read your blogs. Its encourages me to learn sas. Now-a-days i am trying to solve the pi i and pi ij for PPSwor scheme for any size of sample from a population of size N. For that I need the probabilities of all possible samples (or permutations) and that has to be summed up. I can get that for small sample sizes. But for large sample sizes the permutations becomes so bigger. Please help me. With regards,