#### What is an index score?An index score for a Statistical Area Level 1 (SA1) is a weighted combination of Census variables for that SA1. The scores for all SA1s are then standardised to a distribution where the mean equals 1000 and the standard deviation is 100 – this is for convenience of presentation.
For areas larger than SA1s (e.g. Local Government Areas), the scores are a population weighted aggregation of constituent SA1 scores.
It is important to remember that the scores are an ordinal measure, so care should be taken when comparing scores. For example, an area with a score of 1000 is not twice as advantaged as an area with a score of 500. For ease of interpretation, we generally recommend using the index rankings and quantiles (e.g. decile) for analysis, rather than using the index scores.
Note – Prior to SEIFA 2011, CDs were the base unit of geography used in SEIFA (rather than SA1s).
#### What is a rank?To determine the rank of an area, all the areas are ordered from lowest score to highest score. The area with the lowest score is given a rank of 1, the area with the second-lowest score is given a rank of 2 and so on, up to the area with the highest score which is given the highest rank.
#### What is a decile?Deciles divide a distribution into ten equal groups. In the case of SEIFA, the distribution of scores is divided into ten equal groups. The lowest scoring 10% of areas are given a decile number of 1, the second-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.
#### What is a percentile?Percentiles divide a distribution into 100 equal groups. In the case of SEIFA, the distribution of scores is divided into 100 equal groups. The lowest scoring 1% of areas are given a percentile number of 1, the second-lowest 1% of areas are given a percentile number of 2 and so on, up to the highest 1% of areas which are given a percentile number of 100. SEIFA percentiles also allow users to create their own groupings, such as quartiles (which contain 25% of areas).
#### Why do some areas not receive an index score?We try to obtain a reliable index score for as many areas as possible. However, some areas cannot be assigned a score because the population is too small or the quality of the data is not good enough.
#### Which index do I use?Four SEIFA indexes are available, and the choice of index depends on the analysis to be undertaken. Users should consider the aspect of socio-economic advantage and disadvantage in which they are interested, and examine the underlying set of variables in each index. The ABS produces four indexes to allow some flexibility in how they are used.
#### Why does the same area have a different rank for each index?Each index aims to capture a slightly different aspect of relative disadvantage and advantage and is constructed using different variables. It is therefore possible for the same area to have different rankings on each index. For example, it is possible for an area to rank relatively highly in the Disadvantage index, but not in the Advantage and Disadvantage index, because these indexes include different variables.
#### Can I compare SEIFA across time?The indexes are designed to compare the relative socio-economic characteristics of areas at a given point in time, not to compare individual areas across time (longitudinal analysis). When considering longitudinal analysis using indexes from different Census years, there are a number of issues that need to be considered and make the analysis very difficult to interpret:
- the constituent variables and variable weights for the index are likely to have changed;
- the boundaries of the relevant small area(s) may have changed;
- the number of areas changes each release - affecting rankings;
- the distribution of the standardised index values will have changed (e.g. a score of 800 does not represent the same level of disadvantage in different years); and,
- changes in the way the variables are defined e.g. 2006 indexes were calculated using the characteristics of an area’s usual residents, rather than those of the people in the area on Census Night (as was done in previous editions of SEIFA).
For these reasons, it can be very difficult to perform useful longitudinal analysis, and it should not be attempted lightly.
#### Why do SA1s in different deciles have the same score?For SEIFA 2011, the decimal places are not shown for the index scores. Therefore, some SA1s appear to have the same score because the decimal places are not shown.
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