1. In panel data, the regression error: A) should be calculated taking into account heteroskedasticity but not autocorrelation. B) only exists for the case of T > 2. C) is likely to be correlated over time within an entity. D) fits all of the three descriptions above. 2. The concepts of internal validity and external validity are important. Identify which of the following statements are ture: (i) Potential threats to external validity arise from differences between the population and setting studied and those of interest. (ii) The components of internal validity are unbiasedness and consistency of the estimator, and desired significance level of hypothesis testing. (iii) A statistical analysis is internally valid if the hypothesized parameter value is inside the confidence interval. (iv) Applying the analysis from the California test scores to another U.S. state is an example of looking for internal validity. A) (i), (ii) B) (i), (iii) C) (iii), (iv) D) (i), (ii), (iii) 3. The difference between an unbalanced and a balanced panel is that: A) you cannot have both fixed time effects and fixed entity effects regressions. B) for unbalanced panels, the variables are observed for each entity and each time period, but for balanced panels, there are some missing data C) the impact of different regressors are roughly the same for balanced but not for unbalanced panels. D) an unbalanced panel has some missing data for at least one time period for at least one