FEASIBILITY OF DISPROPORTIONATE SAMPLE ALLOCATION IN THE MONTHLY POPULATION SURVEY
The ABS has recently investigated the feasibility of using disproportionate allocation in the redesign for the 2001 Monthly Population Survey (MPS) sample. This investigation follows on from a preliminary investigation into the implications of moving away from a constant sampling fraction within each state/territory (i.e. proportionate allocation).
The current sample design for the MPS adopts a constant sampling fraction within each state/territory. All dwellings in a state/territory are divided into strata which are formed by splitting states and territories into sample design regions and then into area types.
Within each stratum the sample is typically selected through a three stage process. Firstly, collection districts (CDs) are selected from all CDs in the stratum; secondly, blocks are formed for each CD selected in the first stage sample, and a block is selected for inclusion in the sample; and finally, all dwellings within the selected block are listed and a cluster of dwellings are selected from the block. The three stage selection process is designed such that all clusters within the state/territory have the same chance of being selected for the MPS sample.
Proportionate allocation has the advantage that it spreads the sample uniformly across different strata. This ensures that the sample size for each sample design region is roughly equal to a constant fraction of the entire population of the region. The distribution of the sample by area types also reflects that of the population within the state/territory.
"Disproportionate Allocation" involves, for example, taking less sample from the more expensive to enumerate areas, and to compensate, more sample from the cheaper to enumerate areas. Another driver for disproportionate allocation is to select more sample from those areas where labour force characteristics vary highly from one locality to another, and less from those areas which are more homogenous in terms of their labour force characteristics. The cost and variance structures of these areas within a state, and how they change under different sample design configurations, are obtained from the cost and variance models.
A number of improvements have been implemented for the 2001 disproportionate allocation investigation which make the results considerably more conclusive than those from the previous investigation. The most important improvements are firstly that the cost model is now based on comprehensive data which fully reflects costs under the telephone interview methodology, and secondly that the variance model is more robust being based on all possible samples under a large number (50) of different designs. In addition these models now cover all area types.
The recent investigation evaluated a number of approaches for undertaking disproportionate allocation. Based on new cost and variance models to be used in the forthcoming 2001 MPS redesign the study compared two disproportionate options with two proportionate options. All of these methods essentially choose design parameters that minimise national cost subject to satisfying both national and state accuracy constraints.
A paper is currently with ABS management for a decision as to whether to proceed with disproportionate allocation for the 2001 MPS redesign. This paper provides a broad assessment of the issues associated with the introduction of disproportionate allocation. While disproportionate allocation has in theory the potential to provide cost savings, there are a number of other factors that need to be considered, such as the implications for the quality of estimates for smaller states and territories, impacts upon the quality of regional labour force data, impacts upon other household surveys that use the MPS design framework, system costs of implementation and the loss of a robust sample design.
For further information, please contact Daniel Elazar on (02) 6252 6962.
Email: daniel.elazar@abs.gov.au.
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