The quality assurance process means that the census figures are the best estimates we can make of the population. However, they are estimates and therefore subject to margins of error. Standard statistical techniques have been used to calculate these error levels, and therefore produce confidence intervals for the ONC results. A 95 per cent confidence interval is a range within which the true population would fall for 95 per cent of the times the sample survey was repeated. For England and Wales as a whole, the confidence interval on the population estimate is +/- 0.2 per cent. This means that the total census figure has a margin of error of plus or minus 104,000. For local authority areas the percentage margins of error are larger, ranging from 6.1 per cent in Luton to 0.6 per cent in Dudley, East Dorset and Redcar and Cleveland. This is the first time it has been possible to estimate the level of precision for a census with any confidence. It should be noted that as with all statistical analysis these standardised calculations do not capture all sources of variation and there will also be, for example, response, capture and coding errors - see the 2001 Census Quality Report. However, our assessment is that, having made an adjustment for dependency, the ONC results remain the best central estimates possible of the population as at census day 2001.

This page last revised: 01 November 2005