BENCHMARKS AND WEIGHTING METHOD
Weighting is the process of adjusting results from a sample survey to infer results for the total in scope population whether that be persons, income units or households. To do this, a 'weight' is allocated to each sample unit e.g. a person or a household. The weight is a value which indicates how many population units are represented by the sample unit. The first step in calculating weights for each unit is to assign an initial weight, which is the inverse of the probability of being selected in the survey. For example, if the probability of a household being selected in the survey was one in 600, then the household would have an initial weight of 600 (that is, it represents 600 households).
The initial weights are then calibrated to align with independent estimates of the population of interest, referred to as benchmarks. Weights calibrated against population benchmarks ensure that the survey estimates conform to the independently estimated distribution of the population rather than to the distribution within the sample itself. The calibration also includes an adjustment to account for differences in the sample enumerated in each of the four quarters over the year. This is a change from previous cycles of SIH prior to 2015–16 where the quarter adjustment was made to initial weights rather than at the calibration step.
Most of the independent person and household benchmarks are based on demography estimates of numbers of persons and households in Australia. The benchmarks are adjusted to include persons and households residing in private dwellings only and to exclude persons living in very remote areas, and therefore do not, and are not intended to, match estimates of the Australian resident population published in other ABS publications. The demography estimates of persons (estimated resident population - ERP) and households used in SIH 2017–18 are built up from the 2016 Census.
In the 2017–18 SIH, as in previous cycles since 2007–08, all persons in each household were assigned a weight. This differs from the 2005–06 SIH where children aged 0–14 years were not given separate weights, but household counts of the number of children were benchmarked to population totals.
The benchmarks used in the calibration of the final weights for the 2017–18 SIH were:
- number of persons
- by state or territory by age by sex
- in five year age groups up to 80+ years for all states and territories (excluding NT)
- in five year age groups up to 70+ years for the NT
- by state or territory by labour force status ('Employed', 'Unemployed' and 'Not in the labour force') (except NT which does not use labour force status)
- numbers of households
- by state, by indexation quarter by capital city/balance of state (except NT and ACT which only use state)
- by state, by household composition (number of adults (1, 2 or 3+) and whether or not the household contains children) (except NT which only uses whether or not the household contains children)
Estimates produced from the SIH are usually in the form of averages (e.g. average weekly income of couple households with dependent children), or counts (e.g. total number of households that own their dwelling or total number of persons living in households that own their own dwelling). For counts of households, the estimate was obtained by summing the weights for the responding households in the required group (e.g. those owning their own dwelling). For counts of persons, the household weights were multiplied by the number of persons in the household before summing. The SIH collects data on the number of people, including children, in each household but separate records with income and other detailed data were only collected for people 15 years and older.
Average income values are obtained in two different ways, depending on whether mean gross household income or mean equivalised disposable household income is being derived.
Estimates of mean gross household income are calculated on a household weighted basis. They are obtained by multiplying the gross income of each household by the weight of the household, summing across all households and then dividing by the estimated number of households. For example, the mean gross household income of couple households with dependent children is the weighted sum of the gross income of each such household divided by the estimated number of those households.
Estimates of mean equivalised disposable household income are calculated on a person weighted basis. They are obtained by multiplying the equivalised disposable income of each household by the number of people in the household (including children) and by the weight of the household, summing across all households and then dividing by the estimated number of people in the population group.
Averages are obtained by adding the weighted household values, and then dividing by the estimated number of households. For example, the mean gross income of Queensland households is the weighted sum of the gross income of each household divided by the sum of the weights relating to the total number of households within that state.