Xem xét mô hình sau: Yi = β1 + β2Di + ui
với Di = 0 đối với 20 quan sát đầu và Di = 1 đối với 30 quan sát còn lại.
Bạn cũng được cho biết rằng var() = 300.
(a) Bạn giải thích β1 và β2 như thế nào?
(b) Các giá trị trung bình của hai nhóm bằng bao nhiêu?
(c) Bạn tính phương sai của (β1 + β2) như thế nào? Lưu ý: Bạn được cho biết rằng cov(β1, β2) = -15

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