14.35 Regression of weight gain on diet Refer to the previous exercise.

a. Set up indicator variables for protein source and for protein level and specify a regression model with the effects both of protein level and protein source on weight gain.

b. Fit the model in part a and explain how to interpret the parameter estimate for the protein level indicator variable.

c. Show how you could test a hypothesis about beta parameters in the model in part a to analyze the effect of protein source on weight gain.

d. Using the fit of the model, find the estimated mean for each of the six diets. Explain what it means when we say that these estimated means do not allow for interaction between protein level and source in their effects on weight loss.