13.76 Multiple choice: Interpret indicator In the model my = a + b1 x1 + b2 x2, suppose that x1 is an indicator variable for smoking, equalling 1 for smokers and 0 for nonsmokers.

a. We set x1 = 0 if we want a predicted mean without knowing if the person is a smoker or not.

b. The slope effect of x2 is b1 for smokers and b2 for nonsmokers.

c. b1 is the difference between the population mean of y for smokers and nonsmokers.

d. b1 is the difference between the population mean of y for smokers and nonsmokers, for all those subjects having x2 fixed, such as x2 = 10.