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st: problem with heckman procedure - URGENT: please help!!

From   Zeenat Soobedar <[email protected]>
To   [email protected]
Subject   st: problem with heckman procedure - URGENT: please help!!
Date   Tue, 25 Sep 2007 11:10:12 +0100

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

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

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