Midterm Exam 2: ECON 141 Fall 2017
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Midterm Exam 2: ECON 141
Fall 2017
October 25, 2017
Exam Questions
Question 1 (40 points)
All parts have equal weight
Give a brief answer, explanation, and/or mathematical derivation to the ive questions below.
1.a: Suppose you want to study average earnings for male/females. One expert suggests the following regression “Regress earnings on a variable that takes value 1 for males and -1 for females.”Would this suggestion yield misleading results? Yes or no and explain.
1.b: “If two OLS coefficients are not statistically signiicant, then the F-test will not reject the Null Hypothesis of joint signiicance. True or False? Explain.
1.c: Consider the following regression Yi = β(ˆ)1X1;i +β(ˆ)2X2;i + Ui. The corresponding R2 is between 0
and 1. True or False? Explain.
1.d: “A low R2 is an indication that the linearity assumption is incorrect.”True or False? Explain.
1.e: “The OLS estimator is the always the best possible estimator among linear unbiased ones”. True or False? Explain.
Question 2 (40 points)
All parts have equal weight
Consider the following model
Yi = β0 + β1Di + β2Gi + β3GiD i + Ui , (1)
where Yi is the GDP growth of country i; Gi is the government expenditure of country i; and Di takes value 1 if country i is in a recession and 0 otherwise. Also Ui is independent of Di and Gi.
2.a: What is the interpretation of the coefficients β0 , β1 , β2 , β3 ?
2.b: Suppose β(ˆ)1 = 0.10 and β(ˆ)3 = 0.010 and the corresponding standard errors are SE(β(ˆ)1 ) = 0.001 and
SE(β(ˆ)3 ) = 0.010. What can you conclude regarding the signiicance of β1 and β3 ? Can you conclude
anything about the role of recession
2.c: Consider the numbers in 2.b and in addition suppose that the t-statistics corresponding to β1 and β3 are independent of each other.
i. Construct a test statistic that allows you to test for H0 : β1 = β3 = 0.
ii. Would you reject or fail to reject the null hypothesis at 95% conidence? Hint: For a Normal random variable, Z , P (jZj ≥ 2.24) 0.975 and for a F2;1 random variable with 2 degrees of freedom, X , P (X ≥ 3) 0.95.
iii. Describe the intuition behind the result in (ii).
2.d: Suppose that instead of running an OLS regression given by 1, you (incorrectly) run Yi = α0 + α1Di + α2Gi + Vi.
i. Willˆ(α)2 be a consistent estimator of the causal efect of government expenditure on GDP growth? Show your answer.
Question 3 (20 points)
All parts have equal weight
Consider the following regression for wages Y and years of education X above elementary school (i.e., think of X = 0 as elementary school education),
Yi = β0 + β1Xi + β2Xi(2) + Ui , (2)
for any i 2 f1, ...ng. Suppose for now that E[U jX] = 0 and β1 > 0 and β2 > 0.
3.a (i) What is the interpretation of β1 ? (ii) What is the implication of the signs in β1 and β2 ? Hint: It is ine to talk about “ininitesimal changes” for the X, even though it is not formally continuous.
3.b Let c > 0 be the known marginal cost of education. You would like to see if a particular x0 level of education is the level that equates the marginal beneit (expressed in terms of the expected wage) and the marginal cost. Design a test for answering such question. Be precise regarding the null hypothesis and what would be the asymptotic distribution of the test statistic. Hint: No need to write the explicit formula for the test statistic.
3.c Suppose now that E[U jX] 0, but there is an observed variable W (think of it as IQ) such that E[U jX = x, W = w] = β3w2 for any x and w. (i) Will the OLS estimator based on the regression model in 2 yield consistent estimators? Explain (an heuristic explanation will suffice). (ii) How would you use the fact that E[U jX = x, W = w] = β3w2 to construct a regression model that yields consistent estimators?
2023-11-21