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## Deciles

### Area-based deciles

All 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 deciles

An 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 use

The 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

• In TableBuilder, the decile variables are based on the national ranking of areas, not state rankings.
• In TableBuilder, the decile variables can take values from 1 to 10 and also 'not applicable'. The 'not applicable' category refers to people who live in an area that did not receive an index score.
• For population-based deciles, each decile contains approximately 10% of the population. It is approximate because the areas contain different population sizes, and thus it is rarely possible to form groups of areas that contain exactly 10% of the population.
• For the Family and Dwelling databases within TableBuilder Pro, note that family-based and dwelling-based SEIFA deciles have not been included. Area-based and population-based deciles are still included. Users should take this into consideration when interpreting tables from the Family and Dwelling databases; i.e. the SEIFA deciles will not contain equal numbers of families or dwellings respectively.
• More information on the SEIFA variables is provided in the 'Annotation Descriptions' within TableBuilder Pro when a table is created.

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