Adjustment factors are calculated based on the proportional difference found in census data to impute health state prevalence for age groups that the Annual Population Survey (APS) does not cover or where APS data are too sparse. These include the age groups for those under 15 years, as the APS sample does not cover those aged 16 years and under. It also includes the age groups for those aged over 84 years, where for many local areas the APS data are too sparse.

For each of the age groups not covered by the APS, the adjustment factors give the ratio of health state prevalence between the two adjacent age groups in the interpolated census estimates. The introduction of interpolated health state prevalence estimates in the periods between the 2011 and 2021 Censuses means we now have distinct adjustment factors for each period from 2011 to 2013 and 2021 to 2023. This assumes the trajectory in health state prevalence between 2011 and 2021 was linear. 2021 Census data are used as a proxy for the years 2022 and 2023.