Expansion factors are calculated for all ages, but are only applied to certain ages. When this model was originally designed in 1996, only a small number of consecutive ages were under-represented in the Medicare data, and the extent of undercoverage was relatively small. As can be seen in the graph Interstate moves, ratio of Census to Medicare-based, by age, Australia above, both the number of ages under-represented and the extent of this under-coverage, have increased over the last 20 years. In 2016, the possibility of applying all factors greater than 1.0 (rather than limiting to an age range) was considered, however the results were too variable, especially for smaller states.
The original intention of the model was to apply an expansion factor to all ages that experienced under-reporting in the Medicare change of address data - with an upper age limit of 55 years, as data becomes more volatile in older ages. That is, any age (below 55) for which the expansion factor was greater than 1.0. When this model was originally designed (in 1996), only a relatively small number of consecutive ages were under-represented, and only by a relatively small amount.
In 2016, over half the ages had expansion factors greater than 1.0, not necessarily consecutively (see the graph Interstate moves, ratio of Census to Medicare-based, by age, Australia above). To reduce this variability and 'future-proof' the model as much as possible, the age range to which the expansion factors are applied was limited to only addressing the main bulk of the under-reporting, at the Australia level. Note that expansion factors are calculated and applied at the state level - it is only the age range which is determined at the Australia level. This approach produced less variability when applying different Census' expansion factors to the same Medicare data, and also performs well in minimising intercensal difference.
The resulting age range to which the expansion factors were ultimately applied was 17-35 for males, and 17-30 for females. These age ranges cover 91% of under-reporting according to the 2016 Census.