I think you need to take a look at the definition of correlation rather than the SAS implementation first. If n=2 what happens to the formula? Correlation You can also try the correlation formula in Excel and see what it gives you if you input 2 numbers (1 or -1 depending on the numbers). Is your full dataset 18? If so, that's a very small sample size . In general, the smaller the sample size the less reliable the results for all statistical indicators. E.G. If you happen to flip heads twice in a row, you can't assume that the probability of getting heads is 1. ... View more