Refer to Exercise 12.18 and the model relating the mean DDT level E(y) of contaminated fish to x1 = miles captured upstream, x2 = length, and x3 = weight. Now consider a model for E(y) as a function of both weight and species (channel catfish, largemouth bass, and smallmouth buffalo).

a. Set up the appropriate dummy variables for species.

b. Write the equation of a model that proposes parallel straight-line relationships between mean DDT level E(y) and weight, one line for each species.

c. Write the equation of a model that proposes nonparallel straight-line relationships between mean DDT level E(y) and weight, one line for each species.

d. Fit the model, part b, to the data saved in the file. Give the least squares prediction equation.

e. Refer to part d. Interpret the value of the least squares estimate of the beta coefficient multiplied by weight.

f. Fit the model, part c, to the data saved in the file. Give the least squares prediction equation.

g. Refer to part f. Find the estimated slope of the line relating DDT level (y) to weight for the channel catfish species.