Highlights
Questions
1 Counterfactual
Suppose you have a random sample of 1500 Swedish teenagers who have all attended preschool, but di↵er in the amount of total time (in months) in preschool. You want to know if preschool attendance a↵ects schooling results later in life, and regress average grade at the end of high school on number of months in preschool: Gradei = 0 + 1Preschool i + ui. Let’s say you ?nd that 1 =0 .10 and statistically signi?cant, i.e. one extra month of preschool attendance is associated with 0.1 points higher average high school grade. Do you think this e↵ect is causal? Why/why not?
2 Exogeneity
You run the regression Yi = 0 + 1X1i + ui, but the true regression is Yi = 0 + 1X1i + 2X2i + ei.
a) Derive the omitted variable bias. Tip: Start from ˆ 1 p! cov(Xi,Yi) var(Xi) and plug in the true model for Yi. Now you run the regression Yi = 0 + 1Xi + ui, butY causes X too: Xi = 0 + 1Yi + vi
b) Derive the simultaneous causality bias. Tip: Start from ˆ 1 p! cov(Xi,Yi) var(Xi) and plug in the model for Yi.
Standard errors in parentheses.
a) Construct a 90 % con?dence interval for 1 given your estimate ˆ 1.
b) Calculate the t-statistic and the p-value for the two-sided test of the null hypothesis: H0 : 1 =0 vs. H1 : 1 6=0 . Do you reject the null hypothesis at the 5 % level? At the 1 % level? c) ˆ 1 will probably be a biased estimator of the e↵ect on crime rates of increasing the prison population. Why? Do you think ˆ 1 is biased upwards or downwards? Why?
3.Causality assumptions
A municipality in Sweden wants to test the e↵ect of o↵ering long term unemployed a temporary 4 months apprenticeship. Since there is limited funding for this, the municipality randomly assigns 30 % of the unemployed ”old population” to the treatment group and 50 % of unemployed newly arrived immigrants to the treatment group. Those who do not get an apprenticeship o↵er, constitute the control group.
Two years later, the municipality do a follow-up study looking at the wage income of those who were part of the study (treatment and control groups). Let Yi denote the wage for individual i, X1i denote a binary variable that equals 1 if the individual is o↵ered an apprenticeship (and 0 otherwise), and X2i denote a binary variable that equals
a) Consider the regression Yi = 0 + 1X1i + ui. Write down the conditional mean zero assumption.
b) Do you think that the conditional mean zero assumption hold for X1i in a)? Explain.
c) Consider the regression Yi = 0 +1X1i +2X2i +ui. Write down the conditional mean independence assumption for X1i.
d) Does 1 estimate a causal relationship in c)? Does 2 estimate a causal relationship in c)? Explain.
4. Stata assignment
Use the data set CPS92 081 and carry out the following exercises.
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