Your Textbook Modifies the Four Assumptions for the Multiple Regression $\left( \operatorname { cov } \left( u _ { i t } , u _ { i s } \mid X _ { i t } , X _ { i s } \right) = 0 \right.$

Essay

Your textbook modifies the four assumptions for the multiple regression model by adding a new assumption. This represents an extension of the cross-sectional data case, where errors are uncorrelated across entities. The new assumption requires the errors to be uncorrelated across time, conditional on the regressors as well $\left( \operatorname { cov } \left( u _ { i t } , u _ { i s } \mid X _ { i t } , X _ { i s } \right) = 0 \right.$ for $t \neq s$ . (a)Discuss why there might be correlation over time in the errors when you use U.S.state
panel data.Does this mean that you should not use OLS as an estimator?