在help文件里有一段：
Coefficients that occur in one of the models are assumed to be
zero for the other group. It is important, however, that the
associated variables contain nonmissing values for all
observations in both groups.
这个怎么理解？
我的运行错误与这个有关么：
Dependent variable never censored because of selection:
model would simplify to OLS regression