Chemical composition of rainwater. Researchers at the University of Aberdeen (Scotland) developed a statistical model for estimating the chemical composition of water

(Journal of Agricultural, Biological, and Environmental Statistics, Mar. 2005). For one application, the nitrate concentration y (milligrams per liter) in a water sample collected after a heavy rainfall was modeled as a function of water source (groundwater, subsurface flow, or overground flow).

a. Write a model for E(y) as a function of the qualitative independent variable.

b. Give an interpretation of each of the b parameters in the model you wrote in part a.