一个有效的工具变量应满足如下两个条件Acorr(Zi，Xi)=andcorr(Zi，ui)≠Bcorr(Zi，Xi)=andcorr(Zi，ui)=.Ccorr(Zi，Xi)≠andcorr(Zi，ui)≠Dcorr(Zi，Xi)≠andcorr(Zi，ui)=