The information requested falls within the responsibility of the UK Statistics Authority. I have asked the authority to reply.
Letter from Stephen Penneck, dated March 2010:
As Director General for the Office for National Statistics, I have been asked to reply to your recent Parliamentary Question asking what methodology the ONS uses to calculate its Mid-Year Population Estimates (320726).
The ONS produces annual estimates of the resident population of England and Wales as at 30 June each year. ONS Mid-Year Population Estimates are calculated using the cohort component method. This is an established demographic method which is internationally recognised. The method takes the previous year’s population estimate, ages the population by one year, adds in the births and takes away the deaths that have taken place during the year and accounts for both international and internal long-term migration. Adjustments are also made for changes to the numbers of armed forces, prisoners and pupils in boarding schools.
The annotated flow-chart ‘Mid-year Population Estimates: England & Wales’ provides a detailed summary of the cohort component method as used by ONS. A copy has been placed in the House of Commons Library.
The Government, and ONS in particular, have invested heavily in improving the sources and methods used to produce the population estimates. The Migration Statistics Improvement Programme has already delivered substantial benefits, including improved methods for population estimates that will be published in May 2010. These are a result of extensive research by ONS and a large scale engagement programme with users.