无偏性 E(B)=EIXX)XY EI(XX)X(XB+N) EI(XX)XXB+(XX)XNI B+(XX)E(XN) B无偏性 B B X X E X N E X X X XB X X X N E X X X XB N E B E X X X Y = = +   =   +   =   + =   − − − − − ( ) ( ) [( ) ( ) ] [( ) ( )] ) [( ) ] ˆ ( 1 1 1 1 1