约束条件2:随机误差项的方差为 constant PRE: Y,=B,+B2X PRE: Y=B1+B,, X Fig 7-2 (a) Homoscedasticity (equal variance 7-6(b) Heteroscedasticity(unequal variance)7-6 约束条件2：随机误差项的方差为constant Fig 7-2 (a) Homoscedasticity (equal variance); (b) Heteroscedasticity (unequal variance)