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INTRODUCTION

4.1 Estimated resident population (ERP) for customised geography are not standard ABS output, but rather are for satisfying clients who require data for customised geographic areas. They are created by splitting the statistical local area (SLA) ERP into Census collection districts (CDs), then aggregating to the required geography, e.g. postal area, commonwealth electoral district (CED), remoteness area (RA), etc.

4.2 CD ERP are calculated following the creation of SLA ERP (as at 30 June), with preliminary, revised and rebased/final versions of CD ERP prepared accordingly.

4.3 In Census years, CDs are entirely contained within SLAs and do not change boundaries until the following Census. However SLA boundaries can change annually (for example wherelocal government area boundaries change). This means that CDs may be split between two or more SLAs in intercensal years.


Accuracy

4.4 Due to the disaggregation process and the inherent volatility of smaller cells, CD ERP generally does not have the same reliability as SLA ERP.

4.5 It can generally be assumed, however, that the accuracy of CD-based ERP increases with the level of aggregation. ERPs for geographic areas formed by the aggregation of many CDs can generally be regarded as being more reliable than areas that consist of just one or two CDs.


Postal areas

4.6 Postal areas (or CD-derived postcodes) are the most commonly requested customised geography. Postcode boundaries are constructed using whole CDs on a 'best-fit' basis from the most recent Census. This means that some CD-derived postcodes are different to Australia Post postcodes and some Australia Post postcodes are omitted altogether.

4.7 Postal areas exclude non-mappable Australia Post postcodes such as:

• post office box postcodes
• some delivery route postcodes, which are also covered by other postcodes (a situation which often occurs in rural areas)
• some postcodes which, because of the application of the best-fit principle, are not allocated any CDs.