Assignment 6

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New York University *

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Economics

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May 31, 2024

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Assignment 6: ECON-UA 266 - Intro to Econometrics Sahar Parsa Fall 2022 The solution to this assignment will be released on Friday October 28, 5pm. For the Data questions and any other questions that relies on using R, report the output of your analysis in a “report style” pleasing to read and add the codes you used to generate your results. Question 1 Frisch-Waugh Theorem: In the least squares regression of 𝑌 on a constant and one covariate 𝑋 , to compute the regression coeffcients on 𝑋 , we can first transform 𝑌 to deviations from the mean 𝑌 and, likewise, transform 𝑋 to deviations from the column mean; second, regress the transformed 𝑌 on the transformed 𝑋 without a constant. a. What is the OLS estimator if we only transform 𝑋 ? What if we only transform 𝑌 ? b. Do we get the same result? Compare your answers and explain. Question 2 Suppose you estimate the model: 𝑌 = 𝑋𝛽 + 𝜀 using OLS a. Write down the projection matrix P in terms of X , and explain what PY is b. Show the projection matrix P is symmetric and idempotent To prove P is symmetric show P’ = P To prove P is idempotent show PP = P Data Question I From E7.1 (Stock and Watson) + extra questions: Use the Birthweight_Smoking data set (download it on the Week 8 tab on NYU Brightspace) a. To begin, run three regressions: (1) Birthweight on Smoker (2) Birthweight on Smoker, Alcohol, and Nprevist (3) Birthweight on Smoker, Alcohol, Nprevist, and Unmarried b. Application of Frisch-Waughn Theorem 1
Consider regression 2. (1) run a model of Smoker on Alcohol and Nprevist - save the residuals. (2) run a model of Birthweight on Alcohol and Nprevist - save the residuals. (3) run a model of the residuals in 2 on the residuals in 1. (4) Compare the coeffcients in a2 on Smoker to the coeffcient in b3. Are they the same? (5) Explain the result in 4. Why? [This is related to Frisch-Waughn Theorem] c. What is the value of the estimated effect of smoking on birth weight in each of the regressions? d. Does the coeffcient on Smoker in regression (1) suffer from omitted variable bias? Explain. e. Does the coeffcient on Smoker in regression (2) suffer from omitted variable bias? Explain. f. Consider the coeffcient on Unmarried in regression (3). 1. Is the magnitude of the coeffcient large? Explain. 2. A family advocacy group notes that the large coeffcient suggests that public policies that encourage marriage will lead, on average, to healthier babies. Do you agree? (Hint: Review the discussion of control variables in Section 7.5. Discuss some of the various factors that Unmarried may be controlling for and how this affects the interpretation of its coeffcient.) Data Question II Download the earnings and height data from NYU Classes Week 8 tab. a. Run a regression: earnings as dependent variable and height as independent variable. b. From part (a), if your answer is correct, you estimated a relatively large and statistically significant effect of a worker’s height on his or her earnings. One explanation for this result is omitted variable bias: Height is correlated with an omitted factor that affects earnings. For example, Case and Paxson (2008) suggest that cognitive ability (or intelligence) is the omitted factor. The mechanism they describe is straight- forward: Poor nutrition and other harmful environmental factors in utero and in early childhood have, on average, deleterious effects on both cognitive and physical development. Cognitive ability affects earnings later in life and thus is an omitted variable in the regression. Suppose that the mechanism described above is correct. Explain how this leads to omitted variable bias in the OLS regression of Earnings on Height. c. Does the bias lead the estimated slope to be too large or too small? d. Use the years of education variable (educ) to construct four indicator variables for whether a worker has less than a high school diploma ( 𝐿𝑇 _ 𝐻𝑆 =1 if ??𝑢? <12, 0 otherwise), a high school diploma ( 𝐻𝑆 =1 if ??𝑢? = 12, 0 otherwise), some college ( 𝑆𝑜𝑚? _ 𝐶𝑜𝑙 = 1 if 12 < educ <16, 0 otherwise), or a bachelor’s degree or higher ( 𝐶𝑜𝑙𝑙?𝑔? =1 if educ 16, 0 otherwise). Focusing first on women only, run a regression of (1) Earnings on Height and (2) Earnings on Height, including 𝐿𝑇 _ 𝐻𝑆, 𝐻𝑆, and 𝑆𝑜𝑚? _ 𝐶𝑜𝑙 as control variables. 2
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