12.91 Assumptions fail? 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 = year (from 1900 to 2005), y = percentage unemployed workers in the United States. (Hint:

Does y continually tend to increase or decrease?)

b. x = age of subject, y = subject’s annual medical expenses. (Hint: Suppose expenses tend to be relatively high for the newborn and for the elderly.)

c. x = per capita income, y = life expectancy, for nations.

(Hint: The increasing trend eventually levels off.)