Question

Why some where we divide with n and some where we divide with n-1 in t test?

Answer 1

Generally, when one has only a fraction of the population, i.e. a sample, youshould divide by n-1. There is a good reason to do so, we know that thesample variance, which multiplies themean squared deviation from thesample mean by (n−1)/n, is an unbiased estimator of the population variance