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Answers to frequently asked questions about the methodology of population projections

How are projections made?

The national population projections are based on the latest available mid-year population estimates and a set of demographic assumptions about future fertility, mortality and migration based on analysis of trends and expert advice. For further information about NPP methodology, please refer to the NPP methodology document.

The subnational projections take the latest mid-year population estimates as a starting point and add on projected numbers of births, subtract projected numbers of deaths and make an adjustment for net migration. This is carried out at local authority level, and is based upon recent trends in observed numbers of births, deaths and migration. The total over all areas is constrained to the latest national population projections for England. For further information about SNPP methodology, please refer to the SNPP methodology document.

The legal marital status projections are produced using a component methodology that is based on assumptions of underlying marriage, remarriage and divorce rates. Projections of de facto marital status (that is, cohabitation) are produced using assumptions made about the proportions cohabiting in each age/sex/legal marital status group, and how these change over time. The assumptions on proportions cohabiting are then applied to the results of the legal marital status projections to produce projections of cohabiting persons. For further information about marital status projections methodology, please refer to the marital status projections methodology document.

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How are the assumptions underlying the projections agreed?

National-level assumptions about future levels of fertility, mortality and net migration are agreed in liaison with the devolved administrations (namely National Records of Scotland, the Northern Ireland Statistics and Research Agency and the Statistical Directorate of the Welsh Government), following consultation with key users of projections in each country and advice from an expert academic advisory panel. This advisory panel is convened by the British Society for Population Studies, and details of the membership of the panel and minutes of meetings are published online.

Subnational projections and marital status projections use recent data to inform current trends which are assumed to continue over the period of the projection. These projections are constrained to match the national projections, and therefore adopt the same assumptions at the national level. The first-year migration assumptions of the subnational projections are usually consulted upon with key users.

External experts are consulted about the assumptions used in the production of marital status projections to ensure they are appropriate.

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What happens if population estimates are changed/revised? Are projections updated when new estimates are released? Will the projections change when 2011 Census data is released?

Projections are produced with the best available information at the time they are produced. If population estimates have been officially revised or corrected, and there is sufficient time to incorporate the revised figures into calculating the projections, then the projections will be based upon the revised estimates. The same is true when the estimates are updated using Census data.

Each set of projections replaces the previous set. Once projections have been calculated and published, they are neither revised nor updated with new information. New projections may be calculated, or a previous projection may be reproduced as happened with the 2004-based projections, but this would be published as an entirely new projection, not a replacement of an old set.

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Do the projections take government policies into account? What about planned housing growth or economic regeneration?

Population projections are not forecasts and, as such, do not attempt to predict the impact that government policies, development aims, changing economic circumstances nor other factors (whether in the UK or overseas) might have on demographic behaviour. If any recent changes have not yet affected the population estimates or trend data upon which the projections are based, then those changes will not affect the projections. The projections also do not consider the capacity of an area to accommodate changes in its population. They simply provide the population size and age and sex structure that would result if the underlying assumptions about future fertility, mortality and migration were to be realised.

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Content from the Office for National Statistics.
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