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metricsexam2s08sol

metricsexam2s08sol - Econometrics Exam 2 By writing your...

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Econometrics Exam 2 By writing your PID below, you are indicating April 29, 2008 your intention to abide by the honor pledge: PID: This exam consists of ten short questions ( worth four points apiece ) and five long questions ( worth twelve points each ) , for a total of 100 points. You have 120 minutes to answer all completely and clearly in the space provided. Short Questions 1. What is the advantage of using instrumental variables, relative to OLS, and what is the disadvantage? IV is unbiased, regardless of whether the explanatory variables are endogenous. However, it is less efficient than OLS. 2. What would cause the first - difference and fixed - effects estimators to give substantially different results? A failure of “strict exogeneity” causes both estimators to be biased, but biased in different ways so in this case, we would expect FD and FE to give different results. 3. When estimating a system of equations, what would determine whether you use “seemingly unrelated regression” or “three - stage least squares”? SUR is appropriate when all the explanatory variables are exogenous; 3SLS is appropriate when ( some of ) the explanatory variables are endogenous.
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4. What does it mean for an estimator to be “unbiased”? E [ ˆ ! ] = ! 5. List three tests you should perform, if possible, after estimating a model using instrumental variables. The Hausman test; the test for overidentification ( if possible ) , and the test for weak instruments. 6. State the assumptions of the OLS model.
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