概车纶与款理统外 Ex,-6=n2p D(X6)=E(X62)-[E(X)IR m+'m+20, n 故d)m+2g 又n≥2，所以D(,)<D(),较有效.x x n E X n h n ( ) d 1 0 2 +  =   , 2 2  + = n n 2 2 ( ) ( ) [ ( )] D Xh = E Xh − E Xh , ( 1) ( 2) 2 2  + + = n n n , ( 2) 1 ) ˆ ( 2  2  + = n n 故 D ), ˆ ) ( ˆ 2, ( 又n  所以 D  2  D 1 . ˆ ˆ  2 较1 有效