Dear Sir/Madam
I am running a two-step heckman procedure in STATA and I am having this
problem which I have not been able to solve. I hope you can help me:
I am doing the heckman procedure using a loop because I need to do it
for several years
But when STATA executes the command which is as follows:
xi: heckman wf i.agegroup i.qual, twostep select(i.agegroup i.qual
i.agegroup*i.qual ym) mills(mills`y')
/* `y' stands for 1980 1990 2000 */
the program stops and I get the following error: matrix has missing values
r(504);
So I drop the mills ratio generated and then rerun the same command and
then it works. But the thing is that I have to run this one line command
on its own.
Do you have an idea where the problem comes from? Why I cannot run the
code successfully the first time?
Thanks a lot.
Best wishes
Zeenat
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