Intercensal drift in population estimates for England and Wales

Sofie De Broe, Office for National Statistics, UK
Nicola Tromans, Office for National Statistics, UK
Steve Smallwood, Office for National Statistics, UK
Joanne Clements, Office for National Statistics, UK
Julie Jefferies, U.K. Office for National Statistics

One of ONS Centre for Demography’s key challenges is to reduce inter-censal drift in population estimates. This helps add credibility and confidence to the estimates. Inter-censal drift is the accumulation of error in the estimation process. Over time this affects the size and distribution of the population estimates geographically. This paper explores whether fertility rates, sex ratios and mortality measures show evidence of inter-censal drift at the national and sub-national level. The extent to which population drift has occurred since 2001 will not be known until results from the 2011 Census are available. However, inter-censal drift can be observed when comparing the estimates based on the 1991 Census with those re-based on the 2001 Census. This research explores how inter-censal population drift can be observed in demographic measures, such as fertility rates, sex ratios and mortality rates. This is measured by observing the distribution of these measures by geographical area. Evidence suggests that the estimation error increases the spread of these measures over time. This paper then considers lessons to be drawn both for the quality of inter-censal estimates and for the rebasing to the 2011 Census.