WEIGHTING, BENCHMARKING AND ESTIMATION
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 or households. To do this, a 'weight' is allocated to each sample unit i.e. 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 person or household is to assign an initial weight, which is equal to the inverse of the probability of being selected in the survey. For example, if the probability of a person being selected in the survey was 1 in 600, then the person would have an initial weight of 600 (that is, they represent 600 people).
GSS produces statistics on both person level and household level characteristics, and hence both person level weights and household level weights are produced.
Benchmarking
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. Calibration to population benchmarks helps to compensate for over or under-enumeration of particular categories of persons which may occur due to either the random nature of sampling or non-response.
The GSS was benchmarked to the in-scope estimated resident population (ERP) and the estimated number of households in the population. This means that the benchmarks include persons and households residing in private dwellings only and 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 2010 GSS used population and household benchmarks based on the 2006 Census.
Given that the GSS heavily sampled areas of multiple social disadvantage, further analysis was undertaken to ascertain whether benchmark variables, in addition to geography, age, and sex, should be incorporated into the weighting strategy. Analysis showed that the standard weighting approach did not adequately compensate for differential undercoverage in the 2010 GSS sample for variables such as Socio-Economic Indexes for Areas (SEIFA) and labour force status, when compared to other ABS surveys. As these variables were considered to have possible association with social characteristics, additional benchmarks were incorporated into the weighting process.
The benchmarks used in the calibration of final weights for the 2010 GSS were:
- number of persons-
- state by part of state by age by sex; and
- SEIFA; and
- state by labour force status.
- number of households -
- state by part of state by household composition; and
- SEIFA.
Estimation
Each record in the 2010 GSS contains two weights - a person weight and a household weight. The weights indicate how many population units, i.e. persons or households, are represented by the sample unit. In addition, replicate weights have also been included, with 60 person replicate weights and 60 household replicate weights. The purpose of these replicate weights is to enable calculation of the Relative Standard Error (RSE) for each estimate produced.
Survey estimates of counts of persons are obtained by summing the weights of persons or households with the characteristic of interest. Estimates for means, such as mean age of persons, are obtained by summing the weights of persons in each category (e.g. individual ages), multiplying by the value for each category, aggregating the results across categories, then dividing by the sum of the weights for all persons.
For more information on RSEs, please refer to Chapter 5: Data Quality. For more information on use of weights, please refer to Chapter 7: Using the microdata. For a list of the weight variables on the GSS files (person weight, person replicate weights, household weight, household replicate weights), please refer to the survey data item list on the ABS web site <www.abs.gov.au>.
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