(2) D()=ciD(X,)=o'ci 而 1j-+2∑w l≤i<j≤n <∑c+∑c+c)=n i=1 1<i<j<n → >1→pm=g<pa) 1 n 结论 算术均值比加权均值更有效. (2) ∑ ∑ = = = = n i i i n i i D c D X c 1 2 2 1 2 1 (µ ˆ ) ( ) σ n c n i i 1 1 2 ∑ > = ( ˆ ) 1 ( ˆ) 1 2 µ σ D µ n D = < ∑ ∑ ∑ = ≤ < ≤ = < + + = n i i i j n i j n i i c c c n c 1 2 1 2 2 1 2 ( ) 结论 算术均值比加权均值更有效. 而 2 2 1 11 1 2 n n i i i j i i i jn c c cc = = ≤< ≤   = = +     ∑∑ ∑