If you have a population of size n and a sample of size m what is the probability that two values x and y from the population both appear in the sample? So far, I have that each member of the population has probability m/n of being in the sample. Therefore I would guess that the probability that both x and y appear in the sample is (m/n)*(m/n)=m^2/n^2. However the answers show that the probability is (m^2)/n(n-1) but I don't understand why?