An experiment was conducted to study the extrusion process of biodegradable packaging foam.

Source: Data extracted from W. Y. Koh, K. M. Eskridge, and M. A. Hanna, “Supersaturated Split-Plot Designs,” Journal of Quality Technology, 45, January 2013, pp. 61–72.

Among the factors considered for their effect on the unit density (mg/ml) were the die temperature (145°C versus 155°C) and the die diameter (3 mm versus 4 mm). The results were stored in PackagingFoam3. Develop a multiple regression model that uses die temperature and die diameter to predict the unit density (mg>ml). Be sure to perform a thorough residual analysis. Do you think that you need to use both independent variables in the model? Explain.