Homework assignment due mardi, 24 nov

1. Exercise 9.1: Suppose you are interested in the effect of the presence of vending machines in schools on childhood obesity. What randomized experiment would you want to
do (in a perfect world) to evaluate this question?

2. Exercise 9.4 (b,c,d): See statement of Exercise 9.4 from last week's homework assignment. Then:

(b) Is it plausible to believe that these data came from a randomized experiment? Defend your answer.

(c) Another population quantity is the mean of y for those who received the treatment minus the mean of y for those who did not. What is the relation between this quantity and the average treatment effect?

(d) For these data, is it plausible to believe that treatment assignment is ignorable given sex? Defend your answer.

3. Exercise 9.5: For the hypothetical study in the previous exercise, figure out the estimate and the standard error of the coefficient of T in a regression of y on T and x.

4. Exercise 9.6: You are consulting for a researcher who has performed a randomized trial where the treatment was a series of 26 weekly therapy sessions, the control was no therapy, and the outcome was self-report of emotional state one year later. However, most people in the treatment group did not attend every therapy session. In fact there was a good deal of variation in the number of therapy sessions actually attended. The researcher is concerned that her results represent “watered down” estimates because of this variation and suggests adding in another predictor to the model: number of therapy sessions attended. What would you advise her?