Matters needing attention in GMM system state:

1, you must pass two tests, AR 1 is less than 0. 1, and AR is greater than 0.1; Hansen is greater than 0. 1.

2, the code must be robust, many students will not be robust, which is remarkable, but this is the wrong code.

Precautions:

In the setting of dynamic panel model, the lag term of the explained variable is introduced into the regression model as an explanatory variable, which makes the model have dynamic explanatory ability, but the model has endogenous problems. The systematic GMM method estimates both the original horizontal model and the model after differential transformation. System GMM can correct unobserved heteroscedasticity, missing variable deviation, measurement error and potential endogenous problems.