BENCHMARKS AND WEIGHTING
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 1 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 benchmarks used in the calibration of the final weights for the 2005-06 SIH were:
- number of persons aged 15 and over -
- by state or territory by age by sex;
- age groups for the states were 15-19, 20-24, 25-29, 30-34, 35-39, 40-44, 45-49, 50-54, 55-59, 60-64, 65-69, 70-74, 75+
- age groups for the ACT were 15-19, 20-24, 25-29, 30-34, 35-39, 40-44, 45-49, 50-54, 55-59, 60-64, 65-74, 75+
- age groups for the NT were 15-19, 20-24, 25-29, 30-34, 35-39, 40-44, 45+
- by state or the ACT by labour force status ('Employed', 'Unemployed', and 'Not in the labour force');
- by state by capital city/balance of state;
- numbers of children under age 15 -
- by state or territory by age (0-4, 5-14)
- numbers of households -
- by household composition (number of adults (1, 2 or 3+) and whether or not the household contains children)
The benchmark variables used are the same as those used in the 2003-04 SIH. However the age groups used in the 2003-04 SIH for the benchmark relating to the number of persons aged 15 and over by state or territory by age by sex were 15-24, 25-34, 35-44, 45-54, 55-64, 65+. The expanded detail for age groups in SIH 2005-06 aims to improve estimates across all ages, but particularly for older people. The impact of this change on all other estimates not involving age is expected to be minimal.
The person and household benchmarks are based on 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.