Definition Sample Variance $s^2 = \frac{\sum(X_{i}-\bar{X})}{n-1}$ n: size of sample df: n-1 Proof 1. $E(S^2)=\sigma^2$ $E(S^2)$ 가 $\sigma^2$의 점추정치가 될 수 있다. proof) $E(S^2)$ $=E(\frac{1}{n-1}\sum(X_{i}-bar{X})^2)$ $=\frac{1}{n-1}E(\sum(X_{i}-\mu+\mu-\bar{X})^2$ $=\frac{1}{n-1}E(\sum((X_{i}-\mu)^2+(\mu-\bar{X})^2+2(X_{i}-\mu)(\mu-\bar{X})))$ $=\frac{1}{n-1}E(\sum((X_{i}-\mu)^2)+n(\mu-\bar{X})-2n(\..