1. Purpose of variant projections

We produce population projections to understand possible changes in the structure of the population. They are based on assumptions considered to best reflect demographic patterns at the time they are adopted. However, because of the inherent uncertainty of demographic behaviour, any projection will inevitably differ to a greater or lesser extent from actual future population change.

Because of the uncertainty surrounding our main principal projections, we offer a set of alternative scenarios according to higher or lower assumptions about the trajectories of fertility, mortality and migration. We determine our set of variants through consultation with government users. These variants provide an indication of uncertainty and sensitivity to alternative assumptions, but they should not be interpreted as upper or lower limits of future demographic behaviour.

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2. Range of variants

We have published 17 variant projections alongside the principal projections. Some of these are single-component variants, which show the effect of varying one assumption while keeping other assumptions in line with the principal projection; others are combination variants, which look at the effect of varying two or more assumptions.

Table 1 lists the variants published in the 2018-based national population projections (NPPs) and the assumptions underlying each variant. Most of the variants use the principal, high and low assumptions for fertility, mortality and migration; these are summarised numerically in Table 2. Some variants use “special case” assumptions, which are described in greater detail in the Variants that use special case assumptions subsection.

Variants that use special case assumptions

Replacement fertility

Replacement fertility is the level of fertility required for the population to replace itself in size in the long term, given constant mortality rates and in the absence of migration. The replacement level is around 2.075 in the UK, meaning women would need to have, on average, 2.075 children each to ensure the long-term “natural” replacement of the population. The replacement fertility projection combines assumed replacement-level fertility with the principal assumptions of mortality and migration.

Constant fertility

This projection assumes that age-specific fertility rates (ASFRs) will remain constant at the values assumed for the first year of the principal projection, for each constituent country of the UK. It should be noted that for the UK as a whole, the assumption underlying this variant changes slightly through the projection period; this is because the UK assumption is influenced by the way the respective population of each country changes over time. The assumed rates for the first year of the projection are consistent with provisional estimates of total births for the year. The constant fertility projection combines assumed constant fertility with the principal assumptions of mortality and migration.

No mortality improvement

This projection assumes that the combined age- and sex-specific mortality rates remain constant at the values assumed for the first year of the principal projection. It differs from the low mortality assumption in that no mortality improvement assumes an improvement rate of 0.0% from the base year, whereas the low mortality assumption converges to an improvement rate of 0.0% by 2043. The assumed rates are consistent with provisional estimates of total deaths for the year. This projection combines an assumption of no mortality improvement with the principal assumptions of fertility and migration.

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3. Summary of variant projections

In this section, we present a general overview of the different variants and compare them with the principal projection. In particular, we compare projected population sizes for mid-2043 and growth rates over the period from mid-2018 to mid-2043.

Figure 1 and Table 3 show the differences in projected population size for the UK for the principal projection and under the variant scenarios. The 0% future EU migration and 50% future EU migration variants are not included as they are not National Statistics.

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4. Quality and methodology

The National population projections Quality and Methodology Information (QMI) report contains information on the quality characteristics of the projections as well as the methods used to create the data. We have also published information on the decision-making process in National population projections, how the assumptions are set: 2018-based and National population projections, background and methodology: 2018-based, which contain more background detail on each of the fertility, mortality and migration assumptions.

We have produced the 0% and 50% future EU migration variant projections in response to user needs following consultation. These are user-requested statistics first produced for the 2016-based national population projections (NPPs), and we have produced them again for the 2018-based projections. These statistics remain as user-led variant projections produced to meet specific stakeholder requirements, and they are not classed as National Statistics because they have not been created using a standard method.

The EU variant assumptions are produced by applying percentage changes by single year of age and sex to the principal international migration assumption. The percentage changes are calculated based on the proportion of all migration that was to and from the rest of the EU during the last three years of long-term international migration estimates. These changes take effect from the year ending mid-2021 onwards.

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