Seasonally adjusted estimates are derived by estimating and removing systematic calendar related effects from the original series. In most economic data these calendar related effects are a combination of seasonal influences e.g. the weather, social traditions or administrative practices plus other kinds of calendar related variation, such as trading days, Easter or the proximity of significant days (e.g. Christmas).
Institutional effects largely drive the seasonality of the LPI. Important factors are the timing of effect of Australian workplace agreements and certified agreements, the length of these agreements, and the timing of significant wage hearings that impact on rates of pay. A significant institutional change in wage setting arrangements can affect the relative level (or trend) and seasonality of the index, and in recent years there has been some indication of change in the size and timing of wage increases reported.
From the June quarter 2008 release of Labour Price Index, Australia (cat. no. 6345.0) the seasonally adjusted estimates will be derived using autoregressive integrated moving average (ARIMA) modelling. Adoption of ARIMA modelling will reduce the extent of revisions to the seasonally adjusted and trend estimates. For more information on the details of ARIMA modelling see feature article: Use of ARIMA modelling to reduce revisions in the October 2004 issue of Australian Economic Indicators (cat. no. 1350.0).