做动态面板GMM估计的时候,我遇到一个问题:分类变量应当怎么设置?比如我把商业模式作为一个哑变量考虑,这个哑变量有5种情况,那么这个商业模式变量是外生变量,还是内生变量,还是前定变量?这几种情况我都试了试,可是每次都提示如下:note: _Ill_3 dropped because of collinearity
note: _Ill_4 dropped because of collinearity
note: _Ill_2 dropped because of collinearity
note: _Ill_5 dropped because of collinearity
note: _Ill_6 dropped because of collinearity