Understanding the Output

The Seasonal Decomposition procedure creates four new variables for each of the original variables analyzed by the procedure. By default, the new variables are added to the active data set. The new series have names beginning with the following prefixes:

SAF. Seasonal adjustment factors, representing seasonal variation. For the multiplicative model, the value 1 represents the absence of seasonal variation; for the additive model, the value 0 represents the absence of seasonal variation.

SAS. Seasonally adjusted series, representing the original series with seasonal variations removed. Working with a seasonally adjusted series, for example, allows a trend component to be isolated and analyzed independent of any seasonal component.

STC. Smoothed trend-cycle component, which is a smoothed version of the seasonally adjusted series that shows both trend and cyclic components.

ERR. The residual component of the series for a particular observation.

For the present case, the seasonally adjusted series is the most appropriate, because it represents the original series with the seasonal variations removed.

Figure 1. Sequence Charts dialog box
Sequence Charts dialog box

To plot the seasonally adjusted series:

  1. Open the Sequence Charts dialog box.
  2. Click Reset to clear any previous selections.
  3. Right click anywhere in the source variable list, and from the context menu select Display Variable Names.
  4. Select SAS_1 and move it into the Variables list.
  5. Click OK.
Figure 2. Seasonally adjusted series
Seasonally adjusted series

The seasonally adjusted series shows a clear upward trend. A number of peaks are evident, but they appear at random intervals, showing no evidence of an annual pattern.

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