Recently, the ABS has implemented improved methods of producing seasonally adjusted estimates, focused on the application of Autoregressive Integrated Moving Average (ARIMA) modelling techniques. The revision properties of the seasonally adjusted and trend estimates can be improved by the use of ARIMA modelling. ARIMA modelling relies on the characteristics of the series being analysed to project future period data. The projected values are temporary, intermediate values, that are only used internally to improve the estimation of the seasonal factors. The projected data do not affect the original estimates and are discarded at the end of the seasonal adjustment process. This collection will use, from the 2007 annual seasonal reanalysis, concurrent seasonal adjustment with ARIMA modelling where applicable to reduce the level of revision to seasonally adjusted estimates.
A new reference year is typically updated annually every June quarter. From 2007 onwards the updating of the reference year will be completed in the September quarter each year. In September 2007 the new reference year will be 2005-06 for chain volume estimates. This will result in revisions to growth rates in quarters following 2005-06 but will preserve additivity in those quarters. For earlier periods re-referencing affects the levels of, but not the movements in, chain volume estimates.