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).
The initial household weight was then adjusted as described below.
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.
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.
- In 2014-15, 37.7% of children (aged 2-17 years) and 26.8% 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 2014-15 NHS includes both the measured and imputed populations.
- In 2014-15, 24.3% 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 2014-15 NHS includes the valid measurement and imputed populations.
- Non-response rates for physical measurements were higher in 2014-15 than in the 2011-12 NHS; for example, the non-response for BMI for adults in 2014-15 was 26.8% compared with 16.5% in 2011-12.
For further discussion regarding imputation, please refer to the
Imputation section.
An investigation was undertaken to determine whether the characteristics of the people who were measured differed from those who were not measured. This investigation looked at variables such as smoking status, self-assessed health, employment status, marital status, country of birth, self perceived body mass, level of exercise and whether or not has high cholesterol (as a long-term health condition) and found no differences. While there were some differences in age, sex and part of state, these were taken account of in the weighting process.
As the sample weights have been calculated to apply to the whole fully responding sample, use of these weights would produce a correct estimate of the proportion of people with these characteristics (for example, overweight or obesity) but would not produce a correct estimate of the number of people who are overweight or obese. The use of a second weight, to be applied to the measured population only, was considered too confusing for use in microdata products such as TableBuilder and therefore imputation was used to obtain values for respondents for whom physical measurements were not taken and therefore allow the calculation of correct estimates for the number of people who are overweight or obese. |
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 2014
based on the
2011
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 2014-15 NHS survey file were household estimates at 31 December 2014, based on the 2011 Census of Population and Housing,
The household weight was calibrated to the household demography benchmark by State by Part of State by 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 2014-15 NHS survey file.
The person benchmarks used in the person weighting of the 2014-15 NHS survey file were population estimates at 31 December 2014 based on the 2011 Census of Population and Housing.
The
person weight was calibrated to the person demography benchmark by State by Part of state by Age by Sex.
The age groups (years) used for calibration were: 0-1 years; 2-4 years; 5-9 years; 10-11 years; 12-14 years; 15-17 years; 18-19 years; 20-24 years; 25-29 years; 30-34 years; 35-39 years; 40-44 years; 45-49 years; 50-54 years; 55-59 years; 60-64 years; 65-69 years; 70-74 years; 75+ years.
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