各位大神们，使用heckman两步法时恰好第一步probit方程被解释变量是二分变量，且是第二步的主方程的解释变量，这样合理吗？
如下：heckman  y x1 x2 x3 x4 x5 x6 x7 x8 x9,select(x1=x2 x3 x4 x5 x6 )mill(IMR)
为什么使用这个命令时数据结果一直出不来，出现了一直不停跑命令的情况：
Iteration 15990: log likelihood = -127.04346  (not concave)
Iteration 15991: log likelihood = -127.04346  (not concave)
Iteration 15992: log likelihood = -127.04346  (not concave)
Iteration 15993: log likelihood = -127.04346  (not concave)
Iteration 15994: log likelihood = -127.04346  (not concave)
Iteration 15995: log likelihood = -127.04346  (not concave)
Iteration 15996: log likelihood = -127.04346  (not concave)
Iteration 15997: log likelihood = -127.04346  (not concave)
Iteration 15998: log likelihood = -127.04346  (not concave)
Iteration 15999: log likelihood = -127.04346  (not concave)
Iteration 16000: log likelihood = -127.04346  (not concave)

很着急啊，求解答，求解答，感激不尽！