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 regarding what methodology the Office for National Statistics uses to estimate the number of people who permanently leave the UK. (322888)
The Office for National Statistics estimates long-term international emigration based mainly on data from the International Passenger Survey (IPS). Other data sources are used to make adjustments for asylum seekers and their dependants not counted by the IPS, people who change their intentions and, therefore, their migratory status and people who emigrate from Northern Ireland.
The methodology is described in detail at:
These estimates relate to those who intend to leave the UK for at least a year, including those who leave permanently. They are based on people's intentions at the time they leave the UK. Within this we are able to identify the number of people who say they intend to leave for longer than four years, but not those who intend to leave permanently.
The cross-government Migration Statistics Improvement Programme is improving the sources and methods used to produce the international migration estimates. Following extensive research and a large scale engagement programme with users, it has delivered improvements in the accuracy of migration statistics from the International Passenger Survey.