4727.0.55.002 - Australian Aboriginal and Torres Strait Islander Health Survey: Users' Guide, 2012-13
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Contents >> Survey Design and Operation >> Weighting, benchmarking and estimation procedures

WEIGHTING, BENCHMARKS AND ESTIMATION PROCEDURES

Weighting

Weighting is the process of adjusting results from a sample survey to infer results for the total in-scope population. To do this, a 'weight' is allocated to each sample unit; that is, a person or a household. The weight is a value which indicates how many population units are represented by the sample unit. Separate person weights were developed for each survey in the Australian Aboriginal and Torres Strait Islander Health Survey (AATSIHS) and separate household weights were developed for the National Aboriginal and Torres Strait Islander Health Survey (NATSIHS) as outlined below.

• National Aboriginal and Torres Strait Islander Health Survey (NATSIHS)
• Household weight
• Person weight
• Biomedical person weight
• National Aboriginal and Torres Strait Islander Nutrition and Physical Activity Survey (NATSINPAS)
• Person weight
• Biomedical person weight
• Combined NATSIHS/NATSINPAS (referred to as the Core)
• Person weight
• Biomedical person weight
The steps used to derive person and household weights are described below.

Selection weight

The first step in calculating weights for each person or household is to assign an initial weight, which is equal to the inverse of the probability of being selected in the survey. For example, if the probability of being selected in the survey was 1 in 45, then the person would have an initial weight of 45 (that is, they would represent 45 people).

After calculating the initial person weights, an adjustment was incorporated into the weighting to account for Indigenous persons not covered by the sample. The initial household weights (NATSIHS) were also similarly adjusted.

In developing the survey weights, information available for responding and non-responding households was used to conduct quantitative investigations into explicit non-response adjustments. No specific non-response adjustment was made to the weighting however, as the effect of the investigated non-response adjustments to the estimates was negligible after weights were calibrated to population benchmarks (see below).

Investigations were also made into non-response for particular voluntary components where lower levels of response were achieved.

• In 2012-13, 40.4% of respondents aged 18 years and over participated in the biomedical component, NATSIHMS, drawn from respondents in the NATSIHS and NATSINPAS. A biomedical person weight has been produced for each survey file (NATSIHS, NATSINPAS, the Core) in order to adjust for the significant non-response in this voluntary component and to derive the required population estimates. Further analysis of fasting status however indicated that an adjustment or an additional weight was not required in order to produce representative fasting proportion data.
• In 2012-13, 38.7% of NATSINPAS respondents aged 5 years and over participated in the pedometer component. Analysis of the characteristics of people who participated in the pedometer component compared to those who declined indicated that state, remoteness area, and employment status were factors in non-response. However these differences are not associated to the pedometer variables indicating that any potential non-response bias is likely to be small. Further analysis indicated that an adjustment or an additional weight was not required in order to produce representative pedometer steps proportion data.
• In 2012-13, 82.2% of the combined sample from NATSINPAS and NATSIHS aged 2 years and over had their height and weight measured. Therefore, BMI data presented as part of the AATSIHS relates to the measured population only. Analysis of the characteristics of people who agreed to be measured compared to those who declined indicated that age, state, employment status, and remoteness were factors in non-response. Further analysis indicated that an adjustment or an additional weight was not required in order to produce representative BMI data.
• In 2012-13, 79.9% of the combined sample from NATSINPAS and NATSIHS aged 5 years and over agreed to have their blood pressure measured. 78.3% were able to get at least one systolic and diastolic reading and 77.7% had a valid blood pressure reading obtained according to the systolic and diastolic calculation criteria used (see Blood Pressure chapter). Blood pressure data presented as part of the AATSIHS relates to the valid measurement population only.

Benchmarks

Person and household weights are calibrated to independent estimates of the population of interest, referred to as 'benchmarks'. Weights calibrated against population benchmarks ensure that the survey estimates conform to independently estimated distributions of the population rather than to the distribution within the sample itself. Calibration to benchmarks helps to compensate for over- or under-enumeration of particular categories of persons and households, which may occur due to the random nature of sampling, non-response, non-identification or various other undercoverage factors. This process can reduce the sampling error of estimates and may reduce the level of undercoverage bias. For more information on undercoverage, see Data quality page of this Users’ Guide.

All person weights were benchmarked to the Australian Aboriginal and Torres Strait Islander estimated resident population living in private dwellings of Australia at 30 June 2011, based on the 2011 Census of Population and Housing. The benchmarks and hence the estimates from the survey do not (and are not intended to) match estimates of the total Australian Aboriginal and Torres Strait Islander estimated resident population (which include persons in non-private dwellings) obtained from other sources.

A standard approach in ABS household surveys is to calibrate to population benchmarks by state, part of state, age and sex. In terms of the effectiveness of 'correcting' for potential undercoverage bias, it is assumed that the characteristics being measured by the survey for the covered population are similar to the uncovered population within weighting classes, as determined by the benchmarking strategy. Where this assumption does not hold, biased estimates may result.

Household weights

The ABS does not produce Indigenous dwelling counts. Therefore, for the 2012-13 NATSIHS, household level benchmarks were not available. Instead, the household weights for this survey were derived from the person level benchmarks. This was done by assigning the initial household weight (after the adjustment) to all Indigenous persons in the household. These weights were then calibrated to the person level benchmarks with the constraint that each person in the household must have the same final calibrated weight. The resulting weight was assigned as the final household weight. That is, the weights were calibrated in such a way that the household weights will reproduce the number of known person level benchmarks if all people in a household are included. Therefore, the sum of the household weights will only provide an estimate of the number of Indigenous households. This method was then analysed to ensure that person and household level estimates were as consistent as possible.

Note that household weights are only available on the NATSIHS data files, and not on the NATSINPAS or the Core data files.

Person weights

Two types of person weights were produced for the 2012-13 NATSIHS, NATSINPAS and the Core survey files:

• a person weight was applied to all responding persons in the surveys
• a biomedical person weight was applied to all selected persons who provided biomedical samples.

For person level weights, 250 replicate weights were produced to ensure accurate estimates of the standard errors. The age groups (years) available for use in calibration were: 0-1 years, 2-4 years, 5-9 years, 10-14 years, 15-17 years, 18-19 years, 20-24 years, 25-29 years, ... 60-64 years, 65 years and over.

The person weights were simultaneously calibrated to the following population benchmarks:

NATSIHS and the Core files person weights:

• state by remoteness area
• state by age by sex
• Torres Strait Islander status by Torres Strait Islander region by adult/child status
• remoteness area by age by sex
• state by discrete Aboriginal and Torres Strait Islander community.

NATSINPAS file person weight:
• state by remoteness area
• state by age by sex by adult
• Torres Strait Islander status by Torres Strait Islander region by adult/child status
• remoteness area by age by sex
• state by discrete Aboriginal and Torres Strait Islander community.

The biomedical person weights were simultaneously calibrated to the following adult population benchmarks:

NATSIHS file biomedical person weight:

• state by remoteness area
• state by sex
• Torres Strait Islander status by Torres Strait Islander region
• remoteness area by age by sex
• remoteness area by discrete Aboriginal and Torres Strait Islander community.

NATSINPAS file biomedical person weight:
• state
• state by remoteness area
• state by sex
• Torres Strait Islander status by Torres Strait Islander region
• remoteness area by age by sex
• remoteness area by discrete Aboriginal and Torres Strait Islander community.

The Core file biomedical person weight:
• state by remoteness area
• state by sex
• Torres Strait Islander status by Torres Strait Islander region
• remoteness area by age by sex
• remoteness area by discrete Aboriginal and Torres Strait Islander community.

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