EXAMPLE 13.29 Light Rail Usage and Seasonal Ratios

From Example 13.28, we used the moving averages to estimate the level of time series, which is trending over time. For each of the estimated levels given by the centered moving averages, we calculate the ratio of actual sales divided by the centered moving average. For the spreadsheet shown in Figure 13.47, we would enter

in cell G4 and then copy the formula down to cell G20. The resulting ratios are shown in the last column of the Excel spreadsheet.

Because we have more than one seasonal ratio observation for a given quarter, we average these ratios by quarter. That is, we compute the average for all the quarter 1 ratios, then the average for all the quarter 2 ratios, and so on. These averages become our seasonal ratio estimates. The following table displays the seasonal ratios that result.