How accurate are the Census 2001 estimates generally?
ONS are confident that the approach taken for the 2001 Census provides accurate estimates of the population both nationally and for each of the 376 local authorities in England & Wales. All results for each Local Authority area were the subject of intensive quality assurance, which included a comparison with aggregate administrative data such as patient registers, child benefit, schools census and pensions data.

Why do the Census figures differ so much from the Mid Year Estimates?
The Census results now show that the mid-year population estimates have been over-estimating the size of the population and growth in the population over the last few years. There is also evidence that suggests the adjustments made to the 1991 Census were too large, particularly to Inner City and Metropolitan areas.

In the UK, the mid-year population estimates are based on an accepted and widely employed demographic method. Using this method, estimates are produced by updating from a census base allowing for the births, deaths and net migration that has occurred since the census. The most difficult component of population change to estimate accurately is migration. There is no comprehensive registration of migration in the UK. Thus estimates of migration have to be based on survey data and the best proxy data that exist. The critical factor that has been underestimated appears to be emigration. The International Passenger Survey works well, but it captures travellers' intentions at the time of departure and these may be prone to change once people are abroad, particularly among young men with few ties at home. For example Australian immigration control system records many more people born in the UK entering Australia as permanent and long-term settlers than our sources of information. ONS recognises that there is an urgent need to improve some aspects of the migration data which are used to estimate the annual mid-year populations. A National Statistics Quality Review on international migration had already been instigated. This review, which is due to report to the National Statistician by the end of April, will make recommendations for improving international migration statistics over the coming decade. In addition, the National Statistician has initiated a study on the population estimates that is being conducted with the Office of the Deputy Prime Minister and HM Treasury, and which will provide a framework for future improvements.

Consultation with users
The ONS consulted widely on all aspects of the 2001 Census, involving local authorities and many others in this process. The methods were developed with the advice of leading experts from the UK, including from local government, and internationally. Methods were tested extensively in advance and documentation made available for wider comment at the earliest opportunity. The results were subject to an extensive process of quality assurance before publication in accordance with the methodology agreed in advance. The Registrar General was confident, therefore, that the figures published on 30 September 2002 provided the best possible estimate of the population (as defined for the census) for each local authority.

As the Census results were different from the projected population it is expected that the results will be questioned. We have provided the evidence to show why the Census results are accurate estimates of the population, and we are engaging with all of our users to help them understand the methods we used and why these are the best ever Census results.

This page last updated 01 November 2005