A census is the best estimate of the total population. It is the most complete source of information about the population, and produces high-quality estimates to which the annual population estimates are re-based (aligned with) each decade.
Annual population estimates are calculated by updating or ‘rolling forward’ the population from the census base, to account for population change (resulting from births, deaths and migration). These rolled-forward estimates are subject to accumulating uncertainty over the years though, until the next census becomes available and the series is re-based again.
Comparison between the mid-2011 Census-based population estimates and the mid-2011 rolled-forward population estimates (for England and Wales) revealed a 464,000 shortfall. This is the difference that has accumulated in the rolled-forward estimates since the 2001 Census provided the base.
This 464,000 shortfall has been distributed across the decade, to produce revised population estimates for mid-2002 to mid-2010, ensuring a continuous series of population estimates. It is worth noting that these revisions to the population estimates are small compared with the total population: the largest in the series was in the year to mid-2008, at 0.15% of the rolled-forward estimate.
The components of population change
Population change at a national level occurs as a result of two factors: a difference between the number of births and the number of deaths, and a difference between the number of people migrating into a country and the number migrating out of the country (net migration).
Registration data for births and deaths are deemed accurate, so their contribution to population change over the decade has not been revised. This means that the majority of the difference has been attributed to uncertainty in the estimates of migration. As a result, the contribution of net migration to population change during the decade has been revised.
A much smaller portion of the difference has been attributed to other causes which could not easily be quantified (such as uncertainty around the 2001 and 2011 Census-based mid-year estimates used as the start and end points for the series).
Population change due to births, deaths, net migration and other for England and Wales, revised estimates for mid-2001 to mid-2011, with net migration used in the rolled-forward estimates also shown
The chart above shows the contribution that the different types of change have made to the population growth in England and Wales since mid-2001 (that is, the contribution that births minus deaths make compared with net migration). The contribution of migration in the rolled-forward series is also shown to illustrate the impact of the revisions.
It should be noted though that the contributions of births, deaths and migration are interlinked, as past immigrants contribute to the number of births and deaths in subsequent years.
Population change explained
In the year ending mid-2011, the relative contributions of births minus deaths (239,000) and net migration (235,000) were broadly equal. Prior to mid-2008 migration contributed a larger proportion of the population change in each year than the difference between births and deaths. Between mid-2008 and mid-2009 however, the difference between births and deaths had a larger impact on population change than net migration.
The impact of the revisions can be seen by comparing the contributions of net migration to the revised and rolled-forward estimates. Specifically, the larger contributions of migration throughout the decade are concentrated between mid-2005 and mid-2009 in the revised estimates. In this period the revised estimates include additional migrants from the eight countries of Central and Eastern Europe that joined the European Union in 2004 (the EU8) who were missed in the rolled-forward estimates (full details can be found in the Methods paper). The increased level of net migration during this period has resulted in migration maintaining a larger contribution to population change for more of the decade than in the rolled-forward series.
After mid-2009 the difference between the revised and unrevised estimates of net migration is smaller. This reflects the improvements made to the rolled-forward series arising from the Migration Statistics Improvements Programme.
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