■ 회귀모형의 가정들 ※ 추가③ 예측변수들에 대한 가정 ● X1, X2, · · · , Xp are not random variables ● x1j, x2j, · · · , xpj are measured without measurement error for j = 1, 2, · · · , n ● X1, X2, · · · , Xp are linearly independent