HOMEWORK #9 ANSWERS

Chapter 28

1. (a) (i) (b) (iii). See exercises 3-6 on pp. 541ff.

5. (a) With 10 degrees of freedom, P will be bigger. Reason: that curve has more area to the right of 15.

(b) The P-value is bigger when P ² = 15. Reason: the area under the curve to the right of 15 is bigger than the area to the right of 20.

6. Use the method of section 3. The observed frequencies are too close to the expected ones for comfort: P ² » 2 on 10 degrees of freedom, so P < 1% (left tail); by computer, P » 0.4 of 1%. This individual seems to have very good control over the dice. Maybe you should decline his invitation to play craps.

Chapter 29

4. Yes: data snooping (p.549).

Chapter 29 Special Review Exercises

5. False. The data are cross-sectional not longitudinal. An alternative explanation: death rates are higher among people who drink, smoke, and don't eat breakfast. So, fewer of these people survive past 65 and get interviewed.

9. (a) Investigator A gets the higher correlation: B's correlation is attenuated due to restriction of range. (See exercises 4 and 5 on p.130, exercise 9 on p.144, exercise set B on pp.145ff.)

Comment. In the March 1993 Current Population Survey, the correlations were about 0.4 and 0.3, respectively.

(b) The "ecological" correlation-for the state averages-will be higher. See Ch. 9, section 4

12. Something is wrong. The r.m.s. error has to be less than the SD.

19. (a) Each man in the population has had sex with some number of women, perhaps 0. Adding these numbers up gives the total number of female partners for the men. There is a similarly-defined total number of male partners for the women. The two totals must be equal--if a man has a woman as partner, that woman has the man as partner. Since the number of men in the population is about equal to the number of women, the average number of partners must be about the same.

Comment. Strict equality need not hold--as Laumann points out--although the exceptions seem quite minor. For instance, American men might be more likely than women to have sex while travelling out of the country, or to have sex with prostitutes who were unlikely to be sampled in the survey.

(b) The total of a list is related to the average not the median, so Laumann seems to have missed the point.

(c) Lewontin's point is a good one: people in jail, and the homeless, are much more exposed to AIDS than most people. The omission distorts the results in an important way.

(d) Non-respondents may well behave quite differently from respondents.

DISC (Lab Manual)

14. 16.675 years