13.28 You are asked to develop a model to predict the change in the share of consumed energy stemming from renewable resources based on the data provided in the data file Renewable Energy, which contains the data for 26 European countries. The possible predictor variables are the change in the energy tax rates of households and firms, the share of renewable energy in 2004, average GDP/capita growth,

population growth, and the share of environmentally minded households. Find two or three of the best predictor variables from this list.

a. Compute the multiple regression model using the predictor variables selected.

b. Graphically check for heteroscedasticity in the regression errors.

c. Use a formal test to check for heteroscedasticity.