1. Experimental and Quasi-Experimental Research

2. The Tennessee Class Size Experiment
Project STAR (Student-Teacher Achievement Ratio)
4-year study, $12 million.
Upon entering the school system, a student was randomly assigned to one of three groups:
regular class ( students)
regular class + aide
small class ( students)
regular class students re-randomized after first year to regular or regular+aide.
Y = Stanford Achievement Test scores.

3. Deviations from experimental design
Partial compliance:
10% of students switched treatment groups because of “incompatibility” and “behavior problems” - how much of this was because of parental pressure?
Newcomers: incomplete receipt of treatment for those who move into district after grade 1.
Attrition
students move out of district
students leave for private/religious schools

4. Regression analysis The “differences” regression model: where
SmallClassi = 1 if in a small class
RegAidei = 1 if in regular class with aide
Additional regressors (W’s)
teacher experience.
free lunch eligibility.
gender, race.

5. How big are these estimated effects?
Calculate “effect size”, put on same basis by dividing by std. dev. of Y .
Units are now standard deviations of test scores.

6. Direction of bias makes sense:
Without school fixed effects (2), the estimated effect of an additional year of experience is 1.47 (SE = .17).
“Controlling for the school” (3), the estimated effect of an additional year of experience is .74 (SE = .17).
Direction of bias makes sense:
less experienced teachers at worse schools.
years of experience picks up this school effect.
OLS estimator of coefficient on years of experience is biased up without school effects, with school effects, OLS yields unbiased estimator of causal effect.

7. Direction of bias makes sense:
Without school fixed effects (2), the estimated effect of an additional year of experience is 1.47 (SE = .17).
“Controlling for the school” (3), the estimated effect of an additional year of experience is .74 (SE = .17).
Direction of bias makes sense:
less experienced teachers at worse schools.
years of experience picks up this school effect.
OLS estimator of coefficient on years of experience is biased up without school effects, with school effects, OLS yields unbiased estimator of causal effect.

8. Anova: Examples
apple.dta: four different Fertilizers, treatment effects on Weight (in grams);
data.dta: Fifty-eight patients, each suffering from one of three different diseases, were randomly assigned to one of four different drug treatments, and the change in their systolic blood pressure was recorded;
drugs.dta: Repeated records of people assuming drugs and effects of drugs on healthiness.

9. Quasi-Experiment
Data set for the Italian labor market released by ISFOL, an Italian research institute on labor. The ISFOL data under analysis are random sampled from the Italian population in the age range 15-64, in year We aim to compare per capita real labor incomes of different types of work contracts, assessing the impact of the length of the contract. So, in this example, the control population is provided by workers with long term contracts while the treated population are individuals with short term contracts.