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Weighting, benchmarks and estimation procedures

Weighting

Weighting is the 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. Separate person and household weights were calculated, as only one adult and one child per household were enumerated. The steps used to derive person and household weights are described below.

Initial household weight

The first step of the weighting procedure was to assign an initial household weight to fully responding dwellings. The initial household weight was calculated as the inverse of the probability of the household's selection in the sample. For example, if the probability of a household being selected in the survey was 1 in 600, then the household would have an initial weight of 600 (that is, it represents 600 households).

Initial person weights

After obtaining adjusted initial household weights, initial weights were assigned to fully-responding persons based on the sub-sampling scheme deployed within households. Initial person weights were calculated by multiplying the person's household weight by the probability of the person being selected. For persons 18 years and over, the household weight was multiplied by the number of adults aged 18 years and over in the household, and for persons aged 0-17 years old, the household weight was multiplied by the number of children, of selected age applicable to the survey, in the household.

Sample weights have been calculated to apply to the whole fully responding sample. As imputation was used to obtain values for respondents for whom physical measurements were not taken, use of these weights produce correct estimates of the proportion of people with these characteristics (for example, overweight or obese), as well as correct estimates of the number of people with these characteristics. This is similar to the 2014-15 NHS. For comparisons to earlier years, the ABS recommends using proportion comparisons only as imputation was not used on the physical measurement data prior to 2014-15 NHS.

Benchmarks

Person and household weights are calibrated to 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 ensure that the survey estimates conform to independently estimated distributions of the population by age, sex and area of usual residence, 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 or non-response.

All person weights were benchmarked to the estimated resident population living in private dwellings in non-Very Remote areas of Australia at 31 December 2017 based on the 2016 Census of Population and Housing. Excluded from these benchmarks were persons living in 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 estimated resident population (which include persons living in Very Remote areas of Australia and persons in non-private dwellings, such as hotels) obtained from other sources.

Calibration to household level benchmarks

The household benchmarks used in the weighting of the 2017-18 NHS survey file were household estimates at 31 December 2017, based on the 2016 Census of Population and Housing,

The household weight was calibrated to the household demography benchmark by State, part of State and household composition (numbers of persons 0-14 years old, numbers of persons 15 years and over).

Calibration to person level benchmarks

A person weight was produced and applied to all responding persons in the 2017-18 NHS survey file.

Data from the 2017-18 NHS and the 2017-18 Survey of Income and Housing (SIH) was combined to produce the National Health Survey and Survey of Income and Housing pooled dataset (NHIH), in order to enable more accurate smoker status estimates. To preserve consistency between the NHS data and the NHIH dataset, the NHS data was calibrated to the pooled NHIH dataset. This means that unperturbed smoker estimates will be identical between the NHS data and the NHIH data at the benchmark cross-classifications.

The demography benchmarks used in the person weighting of the 2017-18 NHS survey file were population estimates at 31 December 2017 based on the 2016 Census of Population and Housing.

The person weight was calibrated to the person demography benchmark by State, part of State, age, sex and collapsed remoteness area. The person weight was also calibrated to the NHIH pooled dataset by age, sex, area of usual residence and smoker status.

Estimates

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 health conditions) are obtained by multiplying the characteristic of interest with the weight of the reporting person and aggregating.

Non-response adjustment

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 explicit non-response adjustment was made to the weighting however, as the effect of the investigated non-response adjustments to the estimates was negligible.

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

Voluntary measurements of height, weight and waist circumference were taken of respondents aged 2 years and over, while blood pressure was also measured voluntarily for adult respondents (aged 18 years and over). While these items had relatively high non-response rates, analysis indicated no bias existed in the non-responding population. Imputation was used to obtain values for respondents for whom physical measurements were not taken.

In 2017-18, 43.9% of children (aged 2-17 years) and 33.8% of adults (aged 18 years and over) did not have their height, weight or both measured. For these people, height and weight were imputed. BMI data presented as part of the 2017-18 NHS includes both the measured and imputed populations.

• In 2017-18, 31.6% of respondents aged 18 years and over did not have their blood pressure measured. For these people, blood pressure was imputed. Blood pressure data presented as part of the 2017-18 NHS includes the valid measurement and imputed populations.
• Non-response rates for physical measurements were higher in 2017-18 than in the 2014-15 NHS; for example, the non-response for BMI for adults in 2017-18 was 33.8% compared with 26.8% in 2014-15.