∑X E(X=E(=->ECX=-nEC=E(Y) E(S2)=(∑(X-x) NIEy(X-XI D(X) 如果统计量为2≈13 ∑(X,-x),则E(S)≠D(X) 此时,E(S2)DX)2 2 2 1 1 1 1 ( ) ( ( ) ) [ ( ) ] 1 1 n n i i i i E S E X X E X X n n = = = − = − − −   = D X( )2 2 2 1 2 , 1 ( ) ( ) ( ) 1 ( ) ( ) n n i n i n S X X E S D X n n E S D X n = = −  − 如果统计量为  则 此时， ＝ 1 1 1 1 ( ) ( ) ( ) ( ) ( ) n i n i i i X E X E E X nE X E X n n n = = = = = =  