Arsenic in groundwater. Refer to the Environ- mental Science and Technology (January 2005) study of the reliability of a commercial kit to test for arsenic in groundwater, Exercise 4.12 (p. 187). 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, lon- gitude, 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 inter- action model, part a, to the data in the ASWELLS file. Give the least squares pre- diction equation.
(c) Conduct a test (at α = .05) to determine whether latitude and depth interact to effect arsenic level.
(d) Conduct a test (at α = .05) to determine whether longitude and depth interact to effect arsenic level.
(e) Practically interpret the results of the tests, parts c and d.