Often, the distribution of some variants will vary with respect to time. Whether there exists some traditional method or function to measure how a collection of samples sampled in different times follow an identical distribution.
For example, I have two samples, x1, sampled in TIME1, and x2, sampled in TIME2. And the function is f. If the distribution remains the same in both TIME1 and TIME2, then f(x1, x2) = 0; while if the distribution in TIME1 is quite different from that in TIME1, then f(x1, x2) = 1. So what is the f?