Refer to the Environmental Science & Technology (Jan. 2005) study of the reliability of a commercial kit to test for arsenic in groundwater, presented in Exercise. Recall that you fit a first-order model for arsenic level (y) as a function of latitude (x1), longitude (x2), and depth (x3) to data saved in the ASWELLS file.
a. Write a model for arsenic level (y) that includes first-order terms for latitude, longitude, and depth, as well as terms for interaction between latitude and depth and interaction between longitude and depth.
b. Use statistical software to fit the interaction model you wrote in part a to the data in the ASWELLS file. Give the least squares prediction equation.
c. Conduct a test (at α = .05) to determine whether latitude and depth interact to affect arsenic level.
d. Conduct a test (at α = .05) to determine whether longitude and depth interact to affect arsenic level.
e. Interpret practically the results of the tests you conducted in parts c and d.