6553.0 - Survey of Income and Housing, User Guide, Australia, 2013-14  
Latest ISSUE Released at 11:30 AM (CANBERRA TIME) 04/09/2015   
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WEIGHTS


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).

An adjustment is then made to the initial weights to account for differences in the total sample enumerated in each of the four quarters; the sum of the weights across all households in each quarter after this initial adjustment is proportional to the number of days in each quarter. In this survey cycle, as in the 2011–12 Survey of Income and Housing (SIH), the four quarters of survey enumeration have been aligned across the financial year with pension indexation dates rather than calendar quarters to better control sample allocation.

The quarterly adjusted initial weights for SIH 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.

In the 2013–14, all persons in each household were assigned a weight. The 2013–14 SIH was benchmarked to the in scope estimated resident population (ERP) and the projected number of households in the population as of 31 December 2013.
The benchmarks used in the calibration of the final weights for the 2013–14 SIH were:

  • number of persons -
    • by state or territory by age by sex
      • five year age groups up to 80+ years for all states/territories (excluding NT)
      • five year age groups up to 70+ years for the NT
    • by state by capital city/balance of state (excluding NT and ACT which use only state) by quarter of enumeration
    • by state by labour force status ('Employed', 'Unemployed' and 'Not in the labour force') by child/adult
  • number of households -
    • by state by household composition (number of adults (1, 2 or 3+) and whether or not the household contains children) (excluding NT which uses only number of adults 1+).

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 projections of households residing in private dwellings only and to exclude persons living in Very Remote Australia areas, and therefore do not, and are not intended to, match estimates of the Australian resident population published in other ABS publications.

Estimation

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 for income and other detailed data were only collected for people aged 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 only households is the weighted sum of the gross income of couple only household divided by the estimated number of couple only 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. The 'Summary indicators of income distribution' section of this publication illustrates the differences between mean gross household income calculated on a household weighted basis and mean equivalised disposable household income calculated on a person weighted basis.