9.49. Refer to the previous exercise. In view of these assumptions, indicate why such a model would or would not be good in the following situations.

(a) x = time, y = percentage unemployed workers in the United States. (Hint: Does this continually tend to increase or decrease?)

(b) x = income, y = charitable contributions within the previous year. (Hint: Would poor people show as much variation as wealthy people?)

(c) x = age, y = annual medical expenses.

(Hint: Suppose expenses tend to be relatively high for the newborn and for the elderly.)

(d) x = per capita income, y = life expectancy, for nations. (Hint: The increasing trend eventually levels off.)