Consider the monthly warehouse club and superstore sales series discussed in Examples 13.15 and 13.16 (pages 671–674).
club
Fit the trend-seasonal model shown in Example 13.16 and obtain the residuals from the fitted model. Plot the residuals as a time series. Describe the pattern in the residuals. Upon closer inspection, is this pattern evident in the time plot of the sales series shown in Figure 13.28 (page 672)? Explain.
Now consider adding a quadratic trend term to the model. Fit a regression model based on , and the 11 seasonal variables. Is the quadratic term significant?
Based on the estimated model of part (b), obtain the residuals and plot them as a time series. Is there a pattern in these residuals?