Suppose that we have a stochastic process and we suspect that the probability/probability density function changes with time, changes slowly in relation to the sampling rate. Two examples might be: 1. A QM experiment in which the source of identically prepared particles is not quite a stationary process. 2. Repeated rolling of a hypothetical/magical 6-faced die for which the probability for the faces is not uniform and changes slowly with time. From a practical standpoint, how would we measure how fast the overall probability function is changing? (I do not have a strong background in statistics). As always, thanks in advance.