Gauss-Markov Assumptions(cont.) A4.None of the x's is constant,and there are no exact linear relationships among them A5.Assume Var(ux ,x2.....x)=o (Homoskedasticity) Given A1-A5,OLS estimator is BLUE B=(XX)XY Econometrics 15-Zhuxi@SJTU 3Econometrics 15 - Zhuxi@SJTU 3 Gauss-Markov Assumptions(cont.) A4. None of the x’s is constant, and there are no exact linear relationships among them A5. Assume Var(u|x1 , x2 ,…, xk ) = s2 (Homoskedasticity) Given A1-A5, OLS estimator is BLUE.   1 b ˆ X X X Y    