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6 Seasonal adjustment is a means of removing the estimated effects of normal seasonal variation and "trading day effects" from the original series so that the effects of other influences can be more clearly recognised. "Trading day effects" reflect the varying amounts of activity on different days of the week and the different numbers of days of the week in any month (i.e. the number of Sundays, Mondays etc.). Adjustment is also made for Easter which may affect the March and April estimates differently.
7 Seasonal adjustment does not remove from the series the effect of irregular or non-seasonal influences (e.g. the introduction of new models, processing delays, industrial disputes).
8 Irregular influences that are highly volatile can make it difficult to interpret the series even after adjustment for seasonal variation.
9 Extreme care should be exercised in using the seasonally adjusted series for new motor vehicle registrations 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.
10 Seasonally adjusted series are calculated for the ‘Passenger vehicles’, ‘Other vehicles’ and ‘Total vehicles’ series for each State and Territory and are aggregated to obtain the total for Australia.
11 Seasonal adjustment factors are analysed and revised as necessary annually.
12 The smoothing of seasonally adjusted series to create trend estimates reduces the impact of the irregular component of the seasonally adjusted series. The trend estimates are derived by applying a 13-term Henderson-weighted moving average to the respective seasonally adjusted series. These trend series are used to analyse the underlying behaviour of the series over time. As a result of the introduction of the New Tax System, a break in the trend series has been inserted between June 2000 and July 2000. Care should therefore be taken in comparing the series over time.
13 While this smoothing technique enables trend estimates to be produced for the latest month, it does result in revisions to the trend estimates for the most recent months as data for subsequent months become available. Generally subsequent revisions become smaller and after three months, usually have a negligible impact on the series. Changes in the original data and re-estimation of seasonal factors may also lead to revisions to the trend. For more information, see A Guide to Interpreting Time Series - Monitoring 'Trends' - an Overview (Cat. no. 1348.0) or contact the Assistant Director, Time Series Analysis on (02) 6252 6345.
14 For a more detailed break-up of the monthly figures presented here, enquiries should be made to the National Information and Referral Service on 1300 135 070 or to Client Services in any ABS office.
Annual data on the latest total vehicle registrations are published in Motor Vehicle Census, Australia, 31 March 2001 (Cat. no. 9309.0).
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