For the model and data of section 9.2, obtain the panel iv estimator in the PE model by applying the ivregress command to the mean-differenced model with a mean-differenced instrument Hint: For example, for variable x, type by id: egen avex mean(x) followed by summarize x and then generate mdx = x - avex + r (mean). Verify that you get the same estimated coefficients as you would with xtivreg, f e.