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5 The statistics presented in this publication include only a selection of the information collected in the 2011-12 NHS; see the Release Schedule for a list of all 2011-13 AHS releases. A list of data items in the survey is available from the Australian Health Survey: Users' Guide, 2011-13 (cat. no. 4363.0.55.001).
SCOPE OF THE SURVEY
6 The NHS was conducted from a sample of approximately 15,600 private dwellings across Australia.
7 Urban and rural areas in all states and territories were included, while Very Remote areas of Australia and discrete Aboriginal and Torres Strait Islander communities (and the remainder of the Collection Districts in which these communities were located) were excluded. These exclusions are unlikely to affect national estimates, and will only have a minor effect on aggregate estimates produced for individual states and territories, excepting the Northern Territory where the population living in Very Remote areas accounts for around 23% of persons.
8 Non-private dwellings such as hotels, motels, hospitals, nursing homes and short-stay caravan parks were excluded from the survey. This may affect estimates of the number of people with some long-term health conditions (for example, conditions which may require periods of hospitalisation).
9 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.
10 The following groups were excluded from the survey:
11 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 information about the household, such as the income of other household members.
12 Dwellings were selected at random using a multistage area sample of private dwellings. The initial sample selected for the survey consisted of approximately 21,100 dwellings. This was reduced to a sample of 18,355 selected households after sample loss (for example, 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, 15,565 (or 84.8%) were fully or adequately responding, yielding a total sample for the survey of 20,426 persons.
APPROACHED SAMPLE, FINAL SAMPLE AND RESPONSE RATES
13 More information on response rates is available in the Australian Health Survey: Users' Guide, 2011-13 (cat. no. 4363.0.55.001).
14 To take account of possible seasonal effects on health characteristics, the sample was spread randomly across a 12-month enumeration period. Between August and September 2011, survey enumeration was suspended due to field work associated with the 2011 Census of Population and Housing.
WEIGHTING, BENCHMARKING AND ESTIMATION
15 Weighting is a process of adjusting results from a sample survey to infer results for the in-scope total population. To do this, a weight is allocated to each sample unit; for example, a household or a person. The weight is a value which indicates how many population units are represented by the sample unit.
16 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.
17 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.
18 The NHS was benchmarked to the estimated resident population living in private dwellings in non-Very Remote areas of Australia at 31 October 2011. Excluded from these benchmarks were persons living in discrete Aboriginal and Torres Strait Islander communities, as well as a small number of persons living within Collection Districts that include discrete Aboriginal and Torres Strait Islander communities. 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.
19 Survey estimates of counts of persons are obtained by summing the weights of persons with the characteristic of interest. Estimates of non-person counts (for example, number of conditions) are obtained by multiplying the characteristic of interest with the weight of the reporting person and aggregating.
RELIABILITY OF ESTIMATES
20 All sample surveys are subject to sampling and non-sampling error.
21 Sampling error is the difference between 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 Note. Sampling error in this survey is measured by Relative Standard Errors (RSEs). In this publication, estimates with an RSE of 25% to 50% are preceded by an asterisk (e.g. *3.4) to indicate the estimate should be used with caution. Estimates with an RSE over 50% are indicated by a double asterisk (e.g. **0.6) and are considered too unreliable for most purposes.
22 Non-sampling error may occur in any data collection, whether it is based on a sample or a full count such as a census. Non-sampling errors occur when survey processes work less effectively than intended. Sources of non-sampling error include non-response, errors in reporting by respondents or in recording of answers by interviewers, and occasional errors in coding and processing data.
23 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.
24 The following methods were adopted to reduce the level and impact of non-response:
25 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. For example:
26 Long-term health 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, 2007-08 and 2011-12 surveys is based on the 10th revision of the ICD.
27 Country of birth was classified to the Standard Australian Classification of Countries (cat. no. 1269.0).
28 Main language spoken at home was classified according to the Australian Standard Classification of Languages (cat. no. 1267.0).
29 Descriptions for data items such as Body Mass Index and the Kessler Psychological Distress Scale (K10) are included in the Glossary to this publication.
RESULTS OF THE SURVEY
30 Summary results of previous National Health Surveys were published separately in National Health Survey: Summary of Results, Australia, 1989-90, 1995, 2001, 2004-05 and 2007-08 (cat. no. 4364.0).
31 While some changes between estimates from different reference periods 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.
32 Further information about the comparability of data between surveys is in the Australian Health Survey: Users' Guide, 2011-13 (cat. no. 4363.0.55.001).
33 The Census and Statistics Act, 1905 provides the authority for the ABS to collect statistical information, and requires that statistical output shall not be published or disseminated in a manner that is likely to enable the identification of a particular person or organisation. This requirement means that the ABS must take care and make assurances that any statistical information about individual respondents cannot be derived from published data.
34 Some techniques used to guard against identification or disclosure of confidential information in statistical tables are suppression of sensitive cells, random adjustments to cells with very small values, and aggregation of data. To protect confidentiality within this publication, some cell values may have been suppressed and are not available for publication but included in totals where applicable. As a result, sums of components may not add exactly to totals due to the confidentialisation of individual cells.
35 Estimates presented in this publication have been rounded. As a result, sums of components may not add exactly to totals.
36 Proportions presented in this publication are based on unrounded figures. Calculations using rounded figures may differ from those published.
37 ABS publications draw extensively on information provided freely by individuals, businesses, governments and other organisations. Their continued cooperation is very much appreciated; without it, the wide range of statistics published by the ABS would not be available. Information received by the ABS is treated in strict confidence as required by the Census and Statistics Act, 1905.
PRODUCTS AND SERVICES
38 Results for Australia from this release are available in spreadsheet form on the Downloads tab of this release. State and territory data are available on request as a consultancy. Inquiries for this service should be made to the National Information and Referral Service on 1300 135 070.
39 For users who wish to undertake more detailed analysis of the survey data, Survey Table Builder will also be made available in 2013. Survey Table Builder is an online tool for creating tables from ABS survey data, where variables can be selected for cross-tabulation. It has been developed to complement the existing suite of ABS microdata products and services including Census TableBuilder and CURFs. Further information about ABS microdata, including conditions of use, is available via the Microdata section on the ABS web site.
40 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. A list of data items is available from the Australian Health Survey: Users' Guide, 2011-13 (cat. no. 4363.0.55.001).
41 Other ABS publications which may be of interest are shown under the 'Related Information' tab of this release.
42 Current publications and other products released by the ABS are listed on the ABS website. The ABS also issues a daily Release Advice on the website which details products to be released in the week ahead.
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