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Dear Statalist,
I have two questions regarding xtabond2 regression in Stata 13.
I have red both Roodman's papers on how to run the regression and the dangers of too many instruments.
1. I keep on getting the warning which says, "Warning: Twostep estimated covariance matrix of moments is singular. Using a generalized inverse to calculate optimal weighting matrix for twostep estimation.
DifferenceinSargan/Hansen statistics may be negative." How can I overcome this problem? What problem does a singluar covariance matrix of moments cause?
2. I would like to run the regression on subsamples and compare the results.For instance, on pooled crosscountry, then for the Highincome and Lowincome countries separately. Although I attempted to use the [if..] option, it says it's a wrong command.
I truly appreciate any help you may provide me.
Toti.
Oklahoma State University
