A recent article discussed a metal deposition process in which a piece of metal is placed in an acid bath and an alloy is layered on top of it. The business objective of engineers working on the process was to reduce variation in the thickness of the alloy layer. To begin, the temperature and the pressure in the tank holding the acid bath are to be studied as independent variables. Data are collected from 50 samples. The results are organized and stored in Thickness. (Data extracted from J. Conklin, "It's a Marathon, Not a Sprint," Quality Progress, June 2009, pp. 46^49.)
Develop a multiple regression model that uses temperature and the pressure in the tank holding the acid bath to predict the thickness of the alloy layer. Be sure to perform a thorough residual analysis. The article suggests that there is a significant interaction between the pressure and the temperature in the tank. Do you agree?