Specify the Multiple Regression Model That Contains the Difference-In-Difference Estimator

Essay

Specify the multiple regression model that contains the difference-in-difference estimator (with additional regressors). Explain the circumstances under which this model is preferable to the simple difference-in-difference estimator. Explain how the W's can be used to test for randomization. How does the interpretation of the W variables change compared to the differences estimator with additional regressors?