For Table 15.12 on page 481, Table 15.22 shows output for a logistic model treating marijuana use as the response variable and alcohol use and cigarette use as explanatory variables.

(a) Set up dummy variables and report the prediction equation. Interpret the signs of the effects of alcohol use and cigarette use.

(b) Why are the estimates in the table equal to 0 at the second category of each explanatory variable?