2. Estimator for O ee n-(k+1)n-(k+1) E(a2)=E( 2 O k-12. Estimator for 2  2 2 ˆ ( 1) ( 1) i e n k n k   = = − + − +  e e 2 2 ( ) ( ) ˆ 1 e e E E n k    = = − −