Every two years the Government Actuary’s Department [1](GAD) 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 because of the inherent uncertainty of demographic behaviour, to give users an indication of this uncertainty. 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 aside from the principal projection only 12 are published by GAD. These are the six possible 'single component' variants (i.e. varying only 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) and also six selected 'combination' variants: 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 dependant groups – children under 16 and people of pensionable age.
As well as producing the 'standard variants' GAD produce 'special case scenarios', or 'what if' projections to illustrate the consequences of a particular, but not necessarily realistic set of assumptions. In total five sets of special case scenarios will be prepared:
In addition (and finally!), two 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 A and Table B.
On the date of the publication of this paper (20 October 2005) only the six standard variants and the zero migration variant were published. The remaining variants will be published on GAD’s website in November 2005. The General Register Office for Scotland plans to produce a paper in early 2006 comparing all the 2004based variant projections with the principal projection.
More details about all the variants mentioned in this paper can be obtained from GAD’s website [2].
Table A Assumptions for the 2004based principal and seven variant projections for Scotland

Assumptions 
Longterm Fertility (Total Fertility Rate  TFR) 
Life Expectancy Males 
Life Expectancy Females 
Longterm Migration 
Standard variants 
High variant 
1.80 
81.4 
85.1 
+12,500 
Principal 
1.60 
79.1 
83.6 
+4,000 

Low variant 
1.40 
76.8 
82.1 
4,500 

Special case scenario 
Zero migration 
1.60 
79.1 
83.6 
0 
From these assumptions, the following projection variants have been created:


Fertility 
Life expectancy 
Migration 
1 
Principal projection 
Principal 
Principal 
Principal 
Standard 'single component' variants 

2 
High fertility 
High 
Principal 
Principal 
3 
Low fertility 
Low 
Principal 
Principal 
4 
High life expectancy 
Principal 
High 
Principal 
5 
Low life expectancy 
Principal 
Low 
Principal 
6 
High migration 
Principal 
Principal 
High 
7 
Low migration 
Principal 
Principal 
Low 
Special case scenario 

8 
Zero migration 
Principal 
Principal 
Zero 
Links
[1] http://www.gad.gov.uk
[2] http://www.gad.gov.uk/