Panel Data methods are very import in empirical studies. Please answer the followings questions regarding to panel data methods.

(a) Write down the standard unobserved effects panel data model with ?? independent variables.

(b) There are four estimation methods, Pooled OLS, FD, FE, and RE, to estimate panel data models. Write down the "estimating equations" for each of the four common methods.

(c) When is RE similar to Pooled OLS, when is RE similar to FE?