Local authority population estimates are calculated using the cohort component method. In this approach, the previous year’s population is aged-on by one year and then adjusted for births, deaths, net international migration, net internal migration and special populations (such as members of the armed forces and prisoners). The data for these adjustments come from several sources. Births and deaths come from the General Register Office administrative registers. International migration estimates come from the International Passenger Survey, supplemented in the case of in-migration by a range of administrative sources. Data on asylum seekers and their dependants come from the Immigration and Nationality Directorate of the Home Office. Internal migration data are primarily based on the NHS (National Health Service) Patient Register.
Adjustments are also made for special population sub-groups that are not captured in the international and internal migration estimates, for example, members of the armed forces and prisoners.
The estimation process is repeated each year, starting from the 2011 Census base and rolled forward using the cohort component method. Uncertainty from international and internal migration includes accumulated uncertainty from previous years rolled forward, plus new uncertainty for the given year. This means that the uncertainty accumulates over time. The longer the lapse since census, the more uncertainty there will be in the estimates. A fuller description of the methodology for the mid-year population estimates is in ONS (Office for National Statistics) (2016).
Measuring uncertainty in the mid-year population estimates is complex due to the number of components included and because the cohort-component approach relies on a range of estimation procedures and data sources, including census, survey and administrative data. The various sources are subject to sampling and non-sampling errors including capture and recording errors, coverage issues, time lags and intercensal drift. For many sources, we do not have comprehensive and quantifiable information on non-sampling error. Also, sources that are good comparators for the mid-year estimates, or components of them, tend to have been used in their production and so are not independent of the estimation process itself.