" The Three Theorems of Psychohistorical Quantitivity:

The population under scrutiny is oblivious to the existence of the science of Psychohistory.
The time periods dealt with are in the region of 3 generations.
The population must be in the billions (±75 billions) for a statistical probability to have a psychohistorical validity. "

― Isaac Asimov , Foundation (Foundation, #1)