As a modern global humanity, statistics and probability define our world from the smallest particles and their motions, to the movement of people and individual behaviors, to the vastness of space bodies.

My question:
What is the probability of the results of statistical measure only concerning a portion or a part of the large category it proposes to identify?
Or perhaps more precisely…
The probability is X = y, where X is any given statistical significance.
What is the probability of a set of X being relevant to the set that is making a statistic about ?