The other thing is the concept of degrees of freedom, when you use Bessel's formula to calculate the standard deviation of an experiment, there is only 1 measured, in fact, you only need to measure it once to get the result, but you measure it n times, and the extra n-1 times is your free allocation, so the degree of freedom is n-1. In probability theory, the degree of freedom is related to the constraints, i.e., related to the number of equations.