Investigations have indicated that the ABS' seasonal adjustment method is sound and accurately reflects historical seasonal and trading day patterns. Alternative non-data based assumptions could be made in making seasonal adjustments. For example, we could assume there is only a length of month effect in December but even this alternative indicates a decline in seasonally adjusted retail sales between November and December 1998. Nevertheless, the ABS will continue to review the seasonal adjustment methods it uses. One potential area of investigation is revising seasonal adjustment monthly rather than annually.
All time series show some degree of irregularity.
Indications are that the December 1998 estimate of monthly retail turnover had a higher than average irregular component. The ABS publishes trend series to help users assess the underlying trend in the face of irregularities in time series.