## Testing model specification and using the program version of gmm

This post was written jointly with Joerg Luedicke, Senior Social Scientist and Statistician, StataCorp.

The command **gmm** is used to estimate the parameters of a model using the generalized method of moments (GMM). GMM can be used to estimate the parameters of models that have more identification conditions than parameters, overidentified models. The specification of these models can be evaluated using Hansen’s *J* statistic (Hansen, 1982).

We use **gmm** to estimate the parameters of a Poisson model with an endogenous regressor. More instruments than regressors are available, so the model is overidentified. We then use **estat overid** to calculate Hansen’s *J* statistic and test the validity of the overidentification restrictions.

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