HOMEWORK #1 ANSWERS

Chapter 2

No. In the Salk trial, the parents who consented were on the whole better off than the parents who did not consent, and their children were more at risk to begin with (p. 4).

4. (a) They were controlling for age and sex as possible confounders; this is discussed on p.13, with respect to a specific disease--lung cancer.

(b) This is the wrong conclusion to draw. Ex-smokers are a self-selected group, and many people give up smoking because they are sick. So recent ex-smokers include a lot of sick people. (Other epidemiological data suggest that if you quit smoking, you will live longer.)

6. Subjects who did not improve during the first part of the trial probably concluded that they were on the placebo (whether they were or they weren't) and would be switched to the "real" medication during the second part of the trial. This expectation made them improve--the placebo effect.

11. (a) The treatment group consists of those who finished boot camp. The control group consists of other prisoners--including those who do not volunteer, or those who volunteer but do not complete the program.

(b) This is observational. The prisoners decide whether to volunteer for boot camp

and whether to stay in the program or drop out. That is the problem--those who volunteer and stay the course might be quite different from the rest.

(c) False.

Chapter 3

2. (a) There are more at age 1, the histogram is higher at 1 than at 11.

(b) There are more at age 31.

(c) There are more age 35-44, the block has more area.

(d) 50%

4. (a) 25%

(b) 99%

(c) 140-150 mm

(d) 135-140mm

(e) About 5 x 2.1 = 10.5%

(f) 102-103 mm

(g) 117-118 mm is a good guess; the interval is somewhere between 115 and 120 mm.

6. (i) and (ii) not (iii). Reason: With lists (i) and (ii), 25% of the people have heights between 66.5 inches and 67.5 inches; 50% between 67.5 and 68.5 inches; 25% between 68.5 and 69.5 inches. Not so with list (iii).

7. (i) Natural causes. (ii) Trauma. Reason: Young people die of accidents, murder, etc.; old people die of heart disease, cancer, etc.

12. False. There are very few days where the temperature is above 90 degrees; the investigators should have looked at the number of riots divided by the number of days in each temperature range.

DISC (Lab manual)

2. Windows