*ppml trade: "Warning: *variance matrix is nonsymmetric or highly singular"

[Daniel Gomez]

Hi,

I am trying to estimate this gravity equation:

ppml trade PAIR_FE1-PAIR_FE$NTij_1 EXPORTER_TIME_FE* IMPORTER_TIME_FE1-IMPORTER_TIME_FE$NT_yr RTA, noconst


however, I have this error: "Warning: variance matrix is nonsymmetric or highly singular"

What does it mean? and how can I correct it?

Thank you!

[Phil Bromiley]

You didn't get a quick answer. You'll improve your chances of a useful answer by following the FAQ on asking questions - provide Stata code in code delimiters, readable Stata output, and sample data using dataex.

I'd first see if your model works in Stata-provided poisson. If not, then I'd look to cutting back the number of regressors. One way to look at this would be to run the model with regression and see what happens. When the variance is highly singular it can suggest so many variables that you're getting unestimable variances for some variables.