Statement: Office for National Statistics

The Office for National Statistics gave Panorama a detailed statement on the population and migration statistics for Slough and the UK.

The estimated increase in the UK population between 2001 and 2005 was 1.1 million Office for National Statistics

Extracts from the ONS statement are published below.

"The effect of migration on population is complex.

The latest population estimate of 117,500 for Slough in 2005 is 3,000 less than in 2001.

This was mainly due to more people leaving Slough for other addresses in the UK than arrived there.

"We know this from a range of sources, including the records of people registering changes of address with GPs."

Background information

The population of the UK was 60.2 million in mid 2005 and that for Slough was 117,500.

Between mid-2001 and mid-2005, 30,400 residents of Slough moved elsewhere in the UK compared to 21,400 UK residents moving to Slough - a net decrease of 9,000 Office for National Statistics

ONS is undertaking a substantial programme to improve migration and population estimates.

Improved methods for estimating international migration flows at a local level are being implemented in the production of the 2006 mid-year population estimates which will be published on 22 August 2007.

Published estimates for mid-2002 to mid-2005 will also be revised to include the effects of the new methodology. These revisions will be released on the same day.

The estimated increase in the UK population between 2001 and 2005 was 1.1 million, while that for Slough is estimated to have decreased by 3,000.

The effect of the improvements are likely to reduce the estimate of the decrease in the population of Slough to 1,900.

The main reason for the reduction in the population estimate for Slough, is the high level of migration out of Slough to elsewhere in the UK.

Between mid-2001 and mid-2005, 30,400 residents of Slough moved elsewhere in the UK compared to 21,400 UK residents moving to Slough - a net decrease of 9,000.

HOW WE COUNT MIGRATION The Census International Passenger Survey NHS patient records Births and deaths Worker Registration Scheme Read more

These estimates are based on records of individuals re-registering with GPs following a change of address.

ONS has previously estimated relatively high levels of long-term international migration into the area with 6,300 international migrants settling in Slough between mid 2001 and mid 2005 and relatively large numbers of Slough residents leaving the UK, around 4,400.

The net contribution of international migration was to add 2,000 to published population estimates for Slough.

The impact of improved methods being introduced in August is likely to increase this net contribution of international migration to Slough between mid 2001 and mid 2005 to 3,100 (based on an inflow of 5,600 and an outflow of 2,500).

Reliability of estimates

ONS produces population estimates at a national level and on a consistent basis for all local authorities (LAs) in England and Wales.

They are compiled using an internationally respected method (the cohort component method).

Population change in local areas is affected both by internal and international migration.

As previously indicated, internal migration is estimated using actual numbers of patients re-registering with GPs following a change of address.

For this reason, the figures are considered to accurately reflect this type of move.

ONS is however examining whether these re-registrations fully capture all moves and, if not, how to address any issues raised.

ONS uses the United Nations definition of a long-term international migrant in the calculation of the population estimates.

That is someone who enters (or leaves) the country for 12 months or more.

It is widely acknowledged that international migration is the most difficult component of population change to estimate accurately.

This has become increasingly challenging in recent years as the volume of international migration has increased, making this the most important driver of population change nationally.

In particular, ONS is developing estimates of short term international migrants (those staying for up to one year) and national estimates will be published in the Autumn.

Estimates of international migration are considered reliable at a national level.

A sample survey, the International Passenger Survey (IPS), is used in conjunction with actual passenger movements from the Civil aviation Authority, ports and the channel tunnel to produce estimates with a high level of precision.

Further adjustments are made for asylum seekers and other special groups.

Under existing methods, these numbers are distributed to local areas using detailed information collected in the IPS and the Census.

The improvements being implemented primarily include improvements to the distribution of international in and out migrants locally and improved assumptions on the proportion of people who change their intentions in terms of length of stay.

These changes represent a significant improvement in the accuracy of international migration estimates locally.