previous | contents | next

Projected Population of Scotland (2008-based)

Annex D Variant projections and assumptions

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:

• Replacement fertility
• Constant fertility
• No mortality improvement
• Natural change only (or zero migration)

In addition special case projections, based on combinations of these assumptions, will be prepared:

• No change projections – shows what would happen if fertility, mortality and net migration were to remain at current levels
• Stationary projections – shows a population with an unchanging size and age structure using replacement level fertility, constant mortality rates at all ages and zero net migration at all ages

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 (21 October 2009) 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 National Statistics website in November 2009. More details about all the variants mentioned in this paper can be obtained from the National Statistics website: http://www.statistics.gov.uk.

Table D1 Assumptions for the 2008-based principal and seven variant projections for Scotland

Table D2 Variants and Scenario

previous | contents | next