When a cyclical decline occurs, it usually takes a continuation of the tendency for several months before a decline below the same month of the year before becomes apparent. Such comparisons often will not indicate correctly the trend during the preceding six months, which is crucial in most current business analysis. But, as illustrated in the example of the department stores above, it is not informative in the vicinity of turning points. This ensures the annual sum of the seasonally adjusted series is approximately equal to that of the actual series. They are not exactly equal because of incomplete cycles, outlier treatment, etc.
Seasonal adjustment adjusts for patterns that occur within a year.
Annual data is not affected by this sub-annual pattern, so the concept of seasonally adjusting annual data is meaningless. It is possible to produce a trend for a series of annual data. This can be done by a applying a weighted moving average. Statistics New Zealand adjusts quarterly data independently of the monthly adjustment. The monthly actual figures are summed and then seasonally adjusted.