This exercise uses the Lesser–Unsworth data in which seed weight is related to cumulative solar radiation for two levels of exposure to ozone. Assume that “low ozone” is an exposure of .025 ppm and...

This exercise uses the Lesser–Unsworth data in which seed weight is related to cumulative solar radiation for two levels of exposure to ozone. Assume that “low ozone” is an exposure of .025 ppm and that “high ozone” is an exposure of .07 ppm.

(a) Set up X and β for the regression of seed weight on cumulative solar radiation and ozone level. Center the independent variables and include an intercept in the model. Estimate the regression equation and interpret the result.

(b) Extend the model to include an independent variable that is the product term between centered cumulative solar radiation and centered ozone level. Estimate the regression equation for this model and interpret the result. What does the presence of the product term contribute to the regression equation?