The health and disability data used in calculating the adjustment factor were obtained from questions asked in the 2011 Census on self-rated general health and activity limitation because of health problems. 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 are the age groups under 15 (under 1, 1 to 4, 5 to 9, 10 to 14 years), as the APS sample does not cover under-16-year-olds, and the age groups over age 84 (85 to 89, 90 years and over), 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 2011 and 2021 Censuses means we now have distinct adjustment factors for each period from 2011 to 2013 and 2020 to 2022. This assumes the trajectory in health state prevalence between 2011 and 2021 was linear.