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4 The information provided to the ABS and included in the estimates for any month may be revised or corrected in later months. This can occur as a result of corrections made by a provider. Where corrections are made to original data they will be identified by the annotation (r) revised.
5 Seasonally adjusted and trend estimates are produced for 'Passenger vehicles', 'Sports utility vehicles' and 'Other vehicles' for each state and territory. These are then aggregated to obtain 'Total vehicles' for each state and territory, as well as the total for Australia.
6 Seasonal adjustment is a means of removing the estimated effects of normal seasonal variation from the original time series estimates. This is done in order to reveal the underlying non-seasonal features of the series. Seasonally adjusted estimates are derived by estimating and removing systematic calendar related effects only, such as seasonal and trading day influences, from the original estimates. Seasonal adjustment does not aim to remove the irregular or non-seasonal influences from the original series, such as the impact of the introduction of new models or industrial disputes.
7 As a general rule, extreme care should be exercised in using the seasonally adjusted series for new motor vehicle sales in Tasmania, the Northern Territory and the Australian Capital Territory. The small numbers and volatile nature of these data makes reliable estimation of the seasonal pattern very difficult.
8 As happens with all ABS seasonally adjusted series, the seasonal adjustment factors are reviewed and revised on a regular basis, taking into account the latest information in relation to features in the time series. For further information about seasonal adjustment, please refer to Time Series Analysis Frequently Asked Questions, 2003 (cat. no. 1346.0.55.002). Further advice on seasonal adjustment factor revisions will be made in future releases of this publication.
9 The seasonally adjusted estimates in this publication have been produced using a concurrent methodology whereby the seasonal factors are revised each month to take into account the seasonality exhibited by the latest observation. A more detailed review is conducted periodically.
10 The revision properties of the seasonally adjusted and trend estimates have been improved by the use of autoregressive integrated moving average (ARIMA) modelling. ARIMA modelling relies on the characteristics of the series being analysed to project future period data. The ARIMA model is assessed as part of the periodic quality review. 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)
11 For information on interpreting time series, refer to Information Paper: Interpreting Time Series - Are you being misled by the Seasons, 2012 (cat no. 1346.0.55.003).
12 In highly volatile time series, fluctuations that are neither systematic nor predictable can make it difficult to interpret the underlying movement of the series even after adjustment for seasonal variation. The smoothing of seasonally adjusted estimates to produce trend estimates reduces the impact of the volatile component of the seasonally adjusted series. Trend estimates are derived by applying a 13-term Henderson-weighted moving average to the respective seasonally adjusted estimates. These trend estimates are used to analyse the underlying behaviour of the series over time.
13 This smoothing technique enables trend estimates to be produced for the latest month. However, as data for subsequent months become available, the trend estimates for the most recent months are revised. Generally, subsequent revisions become smaller and after three months usually have a negligible impact on the series. Changes in the original data and revisions to seasonal factors may also lead to revisions to the trend. As a result of the introduction of The New Tax System, a break in the monthly trend series has been inserted between June 2000 and July 2000. Care should therefore be taken in comparing the series over time.
For further information, refer to Information Paper: A Guide to Interpreting Time Series - Monitoring Trends, 2003 (cat. no. 1349.0).
EFFECTS OF ROUNDING
14 Figures have been rounded and discrepancies may occur between totals and the sums of the component items.
15 Estimates of movement shown in this publication are obtained by taking the difference of unrounded estimates. The movement is then rounded to one decimal place. Therefore where a discrepancy occurs between the reported movement and the difference of the rounded estimates, the reported movement will be more accurate.
17 For a more detailed breakdown of the original monthly figures presented here, inquiries should be made to the Manager, VFACTS, Federal Chamber of Automotive Industries on (03) 9829 1234. Annual data on total vehicle registrations are published in Motor Vehicle Census, Australia (cat. no. 9309.0).
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