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, March 2005). For one application, the nitrate concentration (milligrams per liter) in a water sample collected after a heavy rainfall was modeled as a function of water source (groundwater, subsurface flow, or over ground flow).

a. Identify the dependent variable, y, for the study.

b. Identify the independent variable and give its type (quantitative or qualitative).