2、关于解释变量的假设 ·确定性假设。X values are fixed in repeated sampling.More technically,X is assumed to be nonstochastic. 注意：“in repeated sampling'”的含义是什么？ ·与随机项不相关假设。The covariances between Xi and ui are zero. c0V(X,4)=0,i=1,2,.,n E(X4)=0,i=1,2,.,n 由确定性假设可以推断。 2、关于解释变量的假设 • 确定性假设。X values are fixed in repeated sampling. More technically, X is assumed to be nonstochastic. 注意：“in repeated sampling”的含义是什么？ • 与随机项不相关假设。The covariances between Xi and μi are zero. 由确定性假设可以推断。 cov( , ) 0, 1,2, , ( ) 0, 1,2, , i i i i X i n E X i n   = = = =