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Every two years the Office for National Statistics (ONS), in consultation with the Registrars General, produces a principal population projection and a number of variant projections, based on alternative assumptions of future fertility, mortality and migration, for the UK and its constituent countries. The variants are produced to give users an indication of the inherent uncertainty of demographic behaviour. There are two distinct types of variant produced: standard variants and special case scenarios.
As well as the principal assumptions, high and low assumptions are prepared for each of the components of population change (fertility, life expectancy and net migration). These are used to generate what are referred to as the standard variants. There are 27 possible combinations of these sets of assumptions although, besides the principal projection, only 12 are published by ONS. These are the six possible single component variants and also six selected combination variants. The single component variants vary one component at a time from the principal assumptions, the purpose being to illustrate plausible alternative scenarios rather than to represent upper or lower limits for future demographic behaviour. The combination variants which are published are those which produce the largest/smallest total population size, the oldest/youngest age structure and the largest/smallest dependency ratios. Dependency ratios show the relationship between the working age population and the two main dependent groups - children under 16 and people of pensionable age.
As well as producing the standard variants ONS produce special case scenarios or what if projections to illustrate the consequences of a particular, but not necessarily realistic set of assumptions. Four sets of special case scenarios are prepared:
In addition special case projections, based on combinations of these assumptions, will be prepared:
More details on the variants referred to in this paper and their assumptions are contained in Table D1 and Table D2 on the next page.
On the date of the publication of this paper (26 October 2011) only the six standard variants, the high and low population combination variants and the zero migration variant were published. The remaining variants will be published on the ONS website in November 2011. More details about all the variants mentioned in this paper can be obtained from the ONS website.
Table D1 Assumptions for the 2010-based principal and nine variant projections for Scotland
|Assumptions||Long-term Fertility (Total Fertility Rate - TFR)||Life Expectancy Males (2035)||Life Expectancy Females (2035)||Long-term Migration|
|Standard variants||High variant||1.90||83.3||86.7||+26,000|
|Special case scenario||Zero migration||1.70||80.9||85.1||0|
Table D2 Variants and Scenario
|Standard 'single component' variants|
|4||High life expectancy||Principal||High||Principal|
|5||Low life expectancy||Principal||Low||Principal|
|Special case scenario|
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