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NOTES EXPLANATORY NOTES INTRODUCTION SCOPE METHODOLOGY RELIABILITY OF ESTIMATES COMPARISON WITH MOTOR VEHICLE CENSUS DATA
CONCEPT OF AVERAGES Many of these tables present data as averages. For all average distance tables except table 12 and all average tonne-kilometres tables, the denominator used in calculating these averages is the estimated number of vehicles that contributed to a particular cell. Table 12 presents average kilometres travelled per vehicle for all registered vehicles including those that travelled zero kilometres. The other tables present more detailed information on actual vehicle use where the denominator used in calculating the average is limited to the estimated number of vehicles that contribute to the particular cell. For example, in table 6 the average kilometres travelled for business purposes in Australia by passenger vehicles was derived by dividing the number of kilometres travelled for business purposes by passenger vehicles by the number of passenger vehicles which reported business travel. In tables 2, 10 and 15 the average rate of fuel consumption for each category is calculated by dividing the total fuel consumption by total kilometres. As the denominators used to calculate the cells of a table are different, the averages along a row cannot be used to derive the total column entry for that row. HISTORICAL COMPARISONS ABS DATA AVAILABLE ON REQUEST RELATED PUBLICATIONS AND PRODUCTS RESPONSE AND NON-RESPONSE, By category
(a) Includes deregistration, out of scope and duplicates. (b) Includes responses that were unusable because of unresolved queries or where the vehicle was sold during the reference quarter and the reported data covered less than 14 days; and non-response where no listing could be found to enable contact by telephone, owner contacted by telephone but response still not secured and refusals. Imputation The need for imputation of unfilled items on the returned questionnaires, as for previous surveys, remained quite high. Imputation is the process whereby a value is generated for missing data items by averaging the responses for similar vehicles which were operating for the reference period. Of the questionnaires returned for 2000, 14% of those reporting some vehicle use needed imputation of one or more items apart from the average rate of fuel consumption. The imputation for average rate of fuel consumption for 2000 was 25%. For the 2001 questionnaires the imputation rates were 15% and 24% respectively. Adjustments The SMVU measures the use of all vehicles registered during the reference year. Because selections are taken from vehicles registered some time before the beginning of each collection period, adjustments and additional selections from New Motor Vehicle Registrations are made to account for the change in size of the registered motor vehicle fleet since the population frame was created. This involved two categories:
These activities occur continuously and the adjustments are made to account for the registrations that are estimated to have been added to the registered vehicle fleet between the population frame date and the reference period. Refer to Technical Note 2: Methodological Review for details of changes made as a result of the review. Fuel estimates - petrol Introduction of lead replacement petrol (LRP) occurred progressively over the 2001 SMVU reference period. As LRP was not identified as a separate category on the collection form it is not known whether data providers categorised LRP as leaded or unleaded petrol. However, the large reduction in leaded petrol suggests that a significant proportion of providers did classify LRP as unleaded. For this reason caution should be taken in interpreting the ratio of leaded to unleaded petrol in 2001. Users should contact the ABS if they have any queries on the quality and reliability of estimates for particular purposes. TECHNICAL NOTE 2: METHODOLOGICAL REVIEW INTRODUCTION A review of the methodology used for the Survey of Motor Vehicle Use (SMVU) was undertaken in 2002 to address data quality issues raised in relation to previously published data. This review identified deficiencies in the SMVU population frame which resulted in the selection of a sample that was not representative of the registered vehicle population. This deficiency has been rectified for the selection of the sample for SMVU 2003. The review identified some minor errors in the adjustments used to account for re-registration. These errors have now been rectified. The review also investigated new vehicle provision calculations. While no errors were identified a number of options to improve these calculations were investigated and implemented. This Technical Note specifically outlines the investigations that led to the identification of the frame deficiency and the post-stratification technique used to correct it. To ensure the SMVU sample was representative of the population, random selection was used within each stratum. For the SMVU, the random selection process allocated a random number to each unit on the frame. To select the sample, the frame was sorted by random number and a start point was randomly selected. A number of units were selected in order, depending on the number of selection units required for a particular stratum. An investigation of the SMVU frame revealed a large number of units which had duplicate random numbers. Duplicate random numbers will not produce a bias in a sample as long as the duplicates contain a random assortment of units. The SMVU frame investigation showed however, that the characteristics of certain variables differed between those units with unique random numbers and those with duplicated random numbers. This was particularly the case with Year of Manufacture. Therefore, the distribution of these variables within the resulting sample was dependant on whether the random start and the units selected incorporated duplicate random numbers. All estimates produced from samples selected under this scenario would contain bias, with the direction of this bias dependant on the inclusion of duplicates. POST-STRATIFICATION The collection of SMVU data for 2000 and 2001 had already been completed before the concerns with the frame were identified. To correct for the unrepresentative sample, a process of post-stratification was used. Post-stratification is a method of stratifying a sample after the responses have been received. It is used to improve the quality of results through stratifying by variables that were not used at the time of sample design. In the case of SMVU the frame investigation identified six variables to be used in the post-stratification. These variables were State, Vehicle type, Year of Manufacture, Body Code, Fuel Class and Number of Cylinders. Once post-stratification was applied to the SMVU data, the weights of each unit were adjusted based on the particular post-stratum of that unit to realign sample totals to be representative of population totals. Both 2000 and 2001 were post-stratified independently and the post-stratification will vary from one year to the next. Caution needs to be taken in making comparisons between 2000 and 2001. Comparisons at the broad level are more reliable than those at the detailed level. Previously published 1998 and 1999 data will be re-estimated using post-stratification and published with post-stratified 2002 SMVU data in late 2003. IMPACT The impact of post-stratification on the estimates for the main data items is summarised in the following table. The size of the change varies by data item. IMPACT OF METHODOLOGICAL REVIEW ON SMVU DATA, Australia
It is important to understand that the percentage change before and after the review can vary significantly between state and vehicle type. Therefore, the percentage change figures in the above table for all vehicles at the national level cannot be used at the state or vehicle type level to calculate the changes due to post-stratification. For 2000 and 2001 SMVU data care should be taken in drawing inferences from changes in data over these two years as movements may be subject to high relative standard errors. Therefore the resulting estimates of movements may not be considered statistically significant. There is also potential for increased volatility in the estimates due to the changes that have been implemented as a result of the methodological review. Users should contact the ABS if they have any queries on the methodological review. GLOSSARY Articulated trucks Motor vehicles constructed primarily for load carrying, consisting of a prime mover which has no significant load carrying area, but with a turntable device which can be linked to a semitrailer. Average load carried Average load carried is calculated by dividing the total weight of loads carried by the number of trips made while carrying a load. B-Doubles A B-Double combination consists of a prime mover towing two semitrailers. The first trailer includes a turntable which links to the second trailer, rather than using a dolly to link the trailers as in road train configurations. Buses Motor vehicles constructed for the carriage of passengers. Included are all motor vehicles with 10 or more seats, including the driver's seat. Business kilometres Distance travelled for hire and reward, or charged to a business expense, or for which an allowance was received. All distances travelled for business purposes, irrespective of actual use, and irrespective of vehicle type, are included in total business kilometres. The laden-unladen dissection of distance travelled for business purposes relates only to freight vehicles, i.e. light commercial vehicles, rigid trucks and articulated trucks. Capital city These areas are based on capital city Statistical Divisions as defined in the Australian Standard Geographical Classification (ASGC) 1996. Sydney — this includes the area bounded by Gosford and Wyong; Hawkesbury and Blue Mountains; Campbelltown, Wollondilly and the Sutherland Local Government Areas. Melbourne — this includes the area bounded by Werribee, Melton, Sunbury, Craigieburn, Whittlesea, Healesville, Warburton, Berwick, Pakenham and the whole of Mornington Peninsula. Brisbane — this includes the area bounded by Caboolture, the eastern part of the Pine Rivers Shire, Redcliffe City, Redland Shire, Beenleigh, Logan City and the City of Ipswich. Adelaide — this includes the area bounded by the Gulf of St. Vincent, the Gawler River and the Mount Lofty Ranges from Gawler to Bridgewater through Kangarilla and Willunga to Sellicks Beach. Perth — this includes the area bounded by Yanchep and Bullsbrook; Warnbro, Keysbrook and Wooroloo. Hobart — this includes the area bounded by New Norfolk; Sorell and Carlton Creek; Brighton and Snug. Darwin — this includes Darwin and suburbs, Palmerston and other areas north of the Howard Springs turn-off. Canberra — this includes all of the Australian Capital Territory. Commodity carried The publication of commodities carried is based on the 10 sectional groupings of the Australian Transport Freight Commodity Classification (ATFCC), with the addition of Tools of Trade. Dolly A device intended to link two semitrailers or a rigid truck and a semitrailer. Freight vehicles Consists of light commercial vehicles, rigid trucks and articulated trucks. Fuel consumption Total fuel consumption is calculated by adding the product of total kilometres travelled and reported average fuel consumption for each vehicle. The average rate of fuel consumption is calculated by dividing the total fuel consumption by total kilometres for each vehicle type. Gross Combination Mass (GCM) Tare weight (i.e. unladen weight) of the motor vehicle and attached trailers, plus their maximum carrying capacity. In the survey, this was obtained for vehicles operated in combination (e.g. a prime mover/semitrailer combination, or a rigid truck/trailer combination). Gross Vehicle Mass (GVM) Tare weight (i.e. unladen weight) of the motor vehicle, plus its maximum carrying capacity. In the survey, this was obtained for buses and rigid trucks not usually towing trailers. Interstate This refers to any travel by vehicles outside their state or territory of registration. Light commercial vehicles Motor vehicles constructed for the carriage of goods and which are less than or equal to 3.5 tonnes GVM. Included are utilities, panel vans, cab-chassis and goods carrying vans (whether four-wheel drive or not). Non-freight carrying trucks Specialist motor vehicles or motor vehicles fitted with special purpose equipment, and having little or no goods carrying capacity, e.g. ambulances, cherry pickers, fire trucks and tow trucks. Other Urban Areas These are based on the Australian Standard Geographical Classification (ASGC) 1996 as being either Statistical Districts with a population greater than 40,000 or clusters of collection districts and other urban areas with a population greater than 40,000, based on the 1996 Population Census. New South Wales — within the areas of Newcastle, Wollongong, Bathurst-Orange, Maitland, Albury (excluding Wodonga), Wagga Wagga, Tweed Heads (excluding Gold Coast), Queanbeyan (excluding Canberra ACT), Lismore, Coffs Harbour, Greater Taree, Shellharbour, Cessnock, Nelson Bay, Port Macquarie and Nowra. Victoria — within the areas of Geelong, Ballarat, Bendigo, Wodonga (excluding Albury), Shepparton and Mildura. Queensland — within the areas of Gold Coast (excluding Tweed Heads), Sunshine Coast, Bundaberg, Rockhampton, Mackay, Townsville, Cairns and Toowoomba. Tasmania — within the areas of Launceston, Burnie, Devonport, Penguin, Ulverstone, Wynyard and Latrobe. This category is not applicable in South Australia, Western Australia, the Northern Territory and the Australian Capital Territory. Passenger vehicles Motor vehicles constructed primarily for the carriage of persons and containing up to nine seats (including the driver's seat). Included are cars, station wagons, four-wheel drive passenger vehicles, passenger vans or mini buses with fewer than 10 seats and campervans. Prime movers Motor vehicles constructed primarily for towing semitrailers. Prime movers have no significant load carrying area but are fitted with a turntable for linking to a semitrailer. Rigid trucks Motor vehicles exceeding 3.5 tonnes GVM, constructed with a load carrying area. Included are normal rigid trucks with a tow bar, draw bar or other non-articulated coupling on the rear of the vehicle. Road trains Motor vehicles comprising a prime mover hauling two or more trailers and employing a dolly or a rigid truck hauling two or more trailers. RSE Relative standard error. The standard error expressed as a percentage of the estimate to which it refers. Semitrailer Trailers designed to impose a substantial load on the towing vehicle, usually via a turntable on a prime mover. Stratification Stratification is the process where a population is divided into homogeneous groups called strata that are non-overlapping, and together comprise the whole population. This technique uses auxiliary information to increase the efficiency of a sample design and units are selected independently within each stratum. Tonne-kilometres Total tonne-kilometres is the aggregation of 'the number of tonnes moved multiplied by the distance travelled in kilometres for each individual vehicle carrying freight'. Note that it is not the aggregation of 'the total number of tonnes moved by total kilometres travelled by all vehicles carrying freight'. Tonnes carried Total tonnes carried is the total weight of goods and freight carried during the survey period. The estimate of annual tonnes carried relates to goods and freight uplifted by vehicles and therefore will overstate the actual physical quantity of goods and freight moved during the survey period to the extent that transhipment occurs (i.e. the transfer of goods and freight from one vehicle to another). Travel to and from work The travel between place of residence and place of work at the beginning and end of all working days, including travel to and from public transport stations.
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