Area-based decilesAll areas are ordered from lowest to highest index score, then the lowest 10% of areas are given a decile number of 1, the next lowest 10% of areas are given a decile number of 2 and so on, up to the highest 10% of areas which are given a decile number of 10. This means that areas are divided up into ten equal sized groups, depending on their index score.
Population-based decilesAn alternative way of defining the deciles is to divide them into ten equal groups based on the number of people living in those areas. These deciles contain an equal number of people (or at least as can be best achieved) in each group, rather than an equal number of areas. These are called population-based deciles.
Advice on which type of decile to useThe choice of area-based or population-based deciles is dependent on the type of analysis being conducted, and it is difficult to give general advice. Whichever type is used, the most important thing is to apply the correct interpretation to the results. The subtle difference in definition needs to be understood.
The population-based deciles represent groups of individuals who live in similarly ranked areas, as opposed to groups of similarly ranked individuals. When using population-based deciles, it is easy to fall into the trap of making statements such as 'the 10% most disadvantaged people', when in fact it is the 10% of the population living in the most disadvantaged areas. Some advantaged people will live in these disadvantaged areas.
When producing population counts by decile (as can be done in TableBuilder), area-based deciles can also be difficult to interpret. This is because there are not equal population counts in each of the area-based deciles (there are equal area counts). This is particularly pronounced when the deciles are defined for larger areas; e.g. Local Governent Areas (LGAs). The more disadvantaged LGAs tend to be located in rural and remote areas with lower populations, so the LGA area-based decile 1 has a much lower population compared to decile 10. This means that when investigating a particular sub-population such as the unemployed, the population spread across the area-based deciles can lead to confusing results unless one recognises how the deciles are defined.
In summary, exercise care when interpreting results from either type of decile. It is worthwhile experimenting with different types of tables (including changing from counts to percentages) to ensure that the correct conclusions are being drawn.
Additional Technical Notes