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3 The statistics presented in this publication are a selection of the information available. Some emphasis has been given in this publication to providing information on the National Health Priority Areas; asthma, diabetes, cardiovascular conditions, cancer, selected musculoskeletal conditions and mental health.
SCOPE OF THE SURVEY
4 The NHS was conducted in a sample of approximately 15,800 private dwellings across Australia. Both urban and rural areas in all states and territories were included, but very remote areas of Australia were excluded. Non-private dwellings such as hotels, motels, hospitals, nursing homes and short-stay caravan parks were not included in the survey.
5 Within each selected dwelling, one adult (aged 18 years and over) and one child were randomly selected for inclusion in the survey. Sub-sampling within households enabled more information to be collected from each respondent than would have been possible had all usual residents of selected dwellings been included in the survey.
6 The following groups were excluded from the survey:
7 Trained ABS interviewers conducted personal interviews with selected residents in sampled dwellings. One person aged 18 years and over in each dwelling was selected and interviewed about their own health characteristics. An adult, nominated by the household, was interviewed about one child in the household. Selected children aged 15-17 years may have been personally interviewed with parental consent. An adult, nominated by the household, was also asked to provide some information about the household, such as the income of other household members.
8 Dwellings were selected at random using a multistage area sample of private dwellings. The initial sample selected for the survey consisted of approximately 20,000 dwellings. This was reduced to a sample of approximately 15,800 after sample loss (e.g. households selected in the survey which had no residents in scope of the survey, vacant or derelict buildings, buildings under construction). Of those remaining dwellings 91% were fully or adequately responding, yielding a total sample for the survey of 20,788 persons.
9 To take account of possible seasonal effects on health characteristics, the sample was spread evenly and randomly across an 11 month enumeration period.
10 The NHS was conducted in both urban and rural areas in all states and territories, but excluded persons living in very remote areas of Australia. The exclusion of these persons would have only a minor impact on any aggregate estimates that are produced for individual states and territories, with the exception of the Northern Territory (NT). The NT records contribute appropriately to national estimates but are insufficient to support reliable estimates for the NT, and as a result, are not shown in this publication.
WEIGHTING, BENCHMARKING AND ESTIMATION
11 Weighting is a process of adjusting results from a sample survey to infer results for the total population. To do this, a weight is allocated to each sample unit. The weight is a value which indicates how many population units are represented by the sample unit.
12 The first step in calculating weights for each person was to assign an initial weight, which was equal to the inverse of the probability of being selected in the survey. For example, if the probability of a person being selected in the survey was 1 in 600, then the person would have an initial weight of 600 (that is, they represent 600 others). An adjustment was then made to these initial weights to account for the time period in which a person was assigned to be enumerated.
13 The weights are calibrated to align with independent estimates of the population of interest, referred to as 'benchmarks', in designated categories of sex by age by area of usual residence. Weights calibrated against population benchmarks compensate for over or under-enumeration of particular categories of persons and ensure that the survey estimates conform to the independently estimated distribution of the population by age, sex and area of usual residence, rather than to the distribution within the sample itself.
14 The survey was benchmarked to the estimated population living in private dwellings in non-very remote areas at 31 December 2007. The benchmarks, and hence the estimates from the survey, do not (and are not intended to) match estimates of the total Australian resident population (which include persons living in very remote areas or in non-private dwellings, such as hotels) obtained from other sources.
15 Survey estimates of counts of persons are obtained by summing the weights of persons with the characteristic of interest. Estimates of non-person counts (e.g. number of medications) are obtained by multiplying the characteristic of interest with the weight of the reporting person and aggregating.
RELIABILITY OF ESTIMATES
16 The estimates provided in this publication are subject to sampling and non-sampling error.
17 Sampling error is the difference between published estimates, derived from a sample of persons, and the value that would have been produced if all persons in scope of the survey had been included. For more information refer to the Technical Notes. In this publication, estimates with a relative standard error of 25% to 50% are preceded by an asterisk (e.g. * 3.4) to indicate the estimate should be used with caution. Estimates with a relative standard error over 50% are indicated by a double asterisk (e.g. ** 0.6) and are considered too unreliable for most purposes.
18 Non-sampling error may occur in any data collection, whether it is based on a sample or a full count such as a Census. Sources of non-sampling error include non-response, errors in reporting by respondents or in recording of answers by interviewers, and errors in coding and processing data.
19 Non-response occurs when people cannot or will not cooperate, or cannot be contacted. Non-response can affect the reliability of results and can introduce a bias. The magnitude of any bias depends on the rate of non-response and the extent of the difference between the characteristics of those people who responded to the survey and those who did not.
20 The following methods were adopted to reduce the level and impact of non-response:
21 By careful design and testing of the questionnaire, training of interviewers, and extensive editing and quality control procedures at all stages of data collection and processing, other non-sampling error has been minimised. However, the information recorded in the survey is essentially 'as reported' by respondents, and hence may differ from information available from other sources, or collected using different methodology. In particular, it should be noted that:
22 Long term medical conditions described in this publication are classified to a classification developed for use in the NHS (or variants of that classification), based on the International Classification of Diseases (ICD). The classification of data from the 2001, 2004-05 and 2007-08 surveys is based on the 10th revision of the ICD.
23 Pharmaceutical medications reported by respondents as used for asthma, arthritis, osteoporosis, heart and circulatory conditions, diabetes/HSL, mental health conditions and related to psychological distress were classified by generic type. The classification used was developed by the ABS for this survey, based on the WHO Anatomical Therapeutic Chemical Classification and the framework (based on organ system and therapeutic drug class) underlying the listing of medications in the Australian Medicines Handbook.
24 Country of birth was classified to the Standard Australian Classification of Countries (cat. no. 1269.0); languages spoken at home were classified according to the Australian Standard Classification of Languages (cat. no. 1267.0).
25 Descriptions for key data items such as BMI, and the Kessler Score, can be found in the Glossary section.
RESULTS OF THE SURVEY
26 Summary results of previous NHS's were published separately in National Health Survey: Summary of Results, Australia, 1989-90, 1995, 2001 and 2004-05 (cat. no. 4364.0). Numerous other publications were released from each of these surveys: see paragraph 33 in these Explanatory Notes.
27 While some movements between the 2004-05 and 2007-08 estimates can be attributed at least in part to conceptual, methodological and/or classification differences, there are some instances where the degree or nature of the change suggests other factors are contributing to the movements, including changes in community awareness or attitudes to certain conditions, changes in common terminology affecting how characteristics are reported/described by respondents, improvements in diagnosis or management of conditions, etc. The degree of change attributable to all these factors relative to the actual change in prevalence cannot be determined from information collected in this survey.
28 Further information about the comparability of data between surveys will be contained in National Health Survey: Users' Guide, Australia 2007-08 (cat. no. 4363.0.55.001) available through the ABS web site
PRODUCTS AND SERVICES
29 Summary results from this survey, compiled separately for each state and the ACT, will be made available in spreadsheet form from the ABS web site <www.abs.gov.au> or on request to the ABS. The Northern Territory (NT) records contribute appropriately to national estimates, but are insufficient to support reliable estimates for the NT, and as a result, are not available in spreadsheet form.
30 For users who wish to undertake more detailed analysis of the survey data, two confidentialised unit record data files (CURFs) are proposed to be available in mid 2009. A Basic CURF will be available on CD-ROM and through the ABS web site <www.abs.gov.au>, while an Expanded CURF (containing more detailed information than on the Basic CURF) will be accessible through the ABS Remote Access Data Laboratory (RADL) system only. Further information about these files, including details of how they can be obtained, and conditions of use, is available on the ABS web site <www.abs.gov.au>.
31 Special tabulations are available on request. Subject to confidentiality and sampling variability constraints, tabulations can be produced from the survey incorporating data items, populations and geographic areas selected to meet individual requirements. These can be provided in printed or electronic form. A list of data items is available from the National Health Survey: Users' Guide, Australia 2007-08 (cat. no. 4363.0.55.001) available through the ABS web site <www.abs.gov.au>. Further information about the survey and associated products can be obtained from the contact officer listed on the front of this publication.
32 Other ABS publications which may be of interest are shown below. These are available via the ABS Health Theme page or at <www.abs.gov.au>.
33 Current publications and other products released by the ABS are listed in the Catalogue of Publications and Products (cat. no. 1101.0). The Catalogue is available from any ABS office or the ABS web site <www.abs.gov.au>. The ABS also issues a daily Release Advice on the web site which details products to be released in the week ahead.
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