E(X2)=(E(X)2+D(X) =[x(E(X)2+D(X)-(E(X)2+D(X)} =m(2+o2)-n2O? ∑(2+2)-(2+ 故2(Xx1-X)不是2的无偏估计量( ) ( ( )) ( ) 2 2 E X = E X + D X n n n 2 2 2 2 ( ) 1  =  + −  − ( ) ( ) 1 2 2 2 2 n n  =   + −  + n 2 2  = − 1 2  n n − = 2   故 不是 2 的无偏估计量。 2 i ( )  n  X − X [( ( )) ( )] [( ( )) ( )]} 1 [ 2 2 E X D X E X D X n =  i + i − +