13.26 In Chapter 11, the regression of retail sales per household on disposable income per household was estimated by least squares. The data are given in Table 11.1, and Table 11.2 shows the residuals and the predicted values of the dependent variable. Use the data file Retail Sales.

a. Graphically check for heteroscedasticity in the regression errors.

b. Check for heteroscedasticity by using a formal test.