14.6 Monthly data for retail auto sales are shown in Table 14.2 on page 438. The data are also plotted in Fig. 14.13.

(a) Use a 6-month centered moving average to smooth the data. Is a seasonal pattern evident? Would you expect auto sales to exhibit seasonal regularities?

(b) Using the original data in Table 14.2, apply the seasonal adjustment procedure described in the text. Plot the 12 final seasonal indices as a function of time and try to explain the shape of the curve. Also plot the seasonally adjusted series and compare it to the original series.