Christian,
Based on Christian's message
(http://www.stata.com/statalist/archive/2008-10/msg00723.html), it seems
that co-variance matrix of estimated parameters has missing values (.). This
could be the results of too many zero observations or underflow issue (where
the calculated number is very small, smaller than -smallestdouble()- and
Stata assigns it as missing value.
In terms of underflow/overflow issue, I have no idea how to correct these
problems, perhaps some users on the list can help.
Best regards,
Minh Nguyen
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