Why do you revise seasonal factors?
There are two reasons that we revise seasonal factors:
We revise factors when we revise the unadjusted data to achieve a better fit to the revised data.
The estimate of a seasonal factor for a given month, say January 2005, is most strongly influenced by the data from surrounding Januaries (especially from 2004 and 2006). In 2005, when the January data for 2006 and later are not available, the seasonal factor estimate for January 2005 will be of reduced quality, unless X-12-ARIMA has calculated good forecasts of data for 2006 and later years and has used them in place of the data that is not yet available. In any case, when future data become available, we use them to obtain improved seasonal factor estimates for the most recent years of the series. These revised factors lead to revised seasonal adjustments of higher quality.
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