On Unbiased Estimation
Unbiased estimation: For the population, the sample mean is an unbiased estimate of the population mean. If the k-order origin distance expectation exists, the k-order origin moment of the sample is also an unbiased estimate, but the center distance is not.
Biased Variance and Unbiased Variance
Biased estimator variance
Unbiased estimator variance
When the sample size tends to infinity, biased estimator and unbiased estimator are equivalent.
The formula for unbiased estimation of variance proves
the popular understanding of why the denominator is n-1.
The degree of freedom, in layman's terms, is the number of unconstrained variables.
Because μ is known, x1,...xn, n variables only need to know n-1 to know the last one, so the degree of freedom is n-1.