When are the results of a seasonal adjustment procedure, or
other smoothing procedure, likely to be of little value? The
diagnostic approach we present offers an answer to this question
and to other questions concerned with the comparison of competing
adjustments. It is based on a straightforward idea. A minimal
requirement of the output of any smoothing or adjustment procedure
is Stability: appending or deleting a small number of series values
should not substantially change the smoothed values- otherwise,
what reliable interpretation can they have? An important related
principle is that, for a given series, if only one of several
plausible signal extraction procedures has a stable output, then
this procedure should be the preferred one for the series. To
implement these principles successfully, the definition of
stability must be made precise in an appropriate way. Our
implementation is focused on multiohcative adjustments produced by
the widely-used X-11 seasonal adjustment procedure, but it will be
clear that the basic ideas are applicable more widely. The
discussion addresses decisions about direct and indirect seasonal
adjustment, trading day adjustment, trends, forecast extension
prior to adjustment and other common adjustment issues.
Adjustment feability, Indirect adjustments, Trading day
adjustments, X-11 and X-ll-ARIMA.
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