National Population Projections: 2014-based extra variants report

Further to the publication of the 2014-based national population projections for the UK and constituent countries on 29 October 2015, this release presents the data for another 7 variant projections based on alternative assumptions of future fertility, mortality and migration.

In order to publish the principal and main variants as early as possible, the publication of the projections is separated into 2 releases.

These extra variant projections provide alternative future scenarios of population change and can be used to illustrate uncertainty and sensitivity to alternative assumptions in future planning and policy formation. For example, the Office for Budget Responsibility use them to inform their long-term fiscal projections published in the fiscal sustainability report.

National population projections are produced for the UK and its constituent countries every 2 years. These projections are based on the 2014 mid-year population estimates and a set of underlying demographic assumptions regarding future fertility, mortality and migration. These terms are explained in the Statistical Bulletin for the first release of these projections. The assumptions are based on the best statistical evidence available at the time and are agreed in liaison with the devolved administrations – Welsh Government, National Records of Scotland (NRS) and Northern Ireland Statistics and Research Agency (NISRA) - following consultation with the main users of projections in each country and advice from an expert academic advisory panel.

In 2014 a new method for setting and applying the cross-border (intra-UK) migration assumptions as rates, rather than fixed numbers of migrants, was introduced and has been applied for the 2014-based projections. More detail is available on these new methods for incorporating cross border migration rates into the UK National Population Projections. (399 Kb Pdf)

Papers outlining the fertility, mortality and migration assumptions adopted for the 2014-based national population projections were published as part of the 29 October 2015 release.

Due to the inherent uncertainty of demographic behaviour, any set of projections will inevitably be proved wrong, to a greater or lesser extent, as a forecast of future demographic events or population structure. In addition to the principal (main or central) projection, variant projections are produced based on alternative assumptions of future fertility, mortality and migration. These variant projections are intended to provide an indication of uncertainty and sensitivity to alternative assumptions; they do not represent upper or lower limits of future demographic behaviour.

Variants can be grouped into 3 types. Single component variants look at the effect of varying 1 assumption at a time from the principal projection. For example, the high fertility variant uses mortality and migration assumptions consistent with the principal projections but assumes a higher rate of fertility. Combination variants assume alternative rates for 2 or more of the assumptions. For instance, the young population variant assumes high fertility, low life expectancy and high net migration, which results in projections with a younger age profile than the principal projection. It is also sometimes useful to prepare special case scenarios or "what if" projections, to illustrate the consequences of a particular, but not necessarily realistic, set of assumptions, such as zero net migration (natural change only) or no change.

A full list of variants with their associated assumptions is available in Appendix A.

Underlying data for all variant projections in this release can be found in the reference tables and open data tables.

The report on quality and methodology (290.9 Kb Pdf) has more information on how our population projections meet user needs and their fitness for purpose, which includes strengths and limitations.

The principal projection and 9 standard variant projections were published on 29 October 2015. These included 6 possible "single component" variants, 2 "combination" variants producing the largest and smallest total population size, and 1 special case scenario of zero net migration (natural change only).

Variant and principal projections are available for the UK, England, Wales, Scotland and Northern Ireland. The principal projections are also produced for Great Britain and England and Wales as a whole. The high migration, low migration, zero net migration (natural change only), young, and old age population projections are also available at the Great Britain level.

The following 7 additional standard "combination" variant projections and special case scenarios are included in this release.

Standard "combination" variants:

• old age structure
• young age structure

Special case scenarios:

• replacement fertility projection
• constant fertility projection
• no mortality improvement projection
• no change projection
• long-term balanced net migration projection (available for the UK only)

Appendix A has the full list of variants available and their publication dates.

Standard "combination" variants

Old age structure

This projection combines assumptions of low fertility, high life expectancy and low net migration. This results in fewer babies being born, an increase in the number of older persons and a reduction in net migration, which impacts the younger adult age groups the most. This leads to a population with an older age structure than the principal projection.

Young age structure

This projection combines assumptions of high fertility, low life expectancy and high net migration. This results in an increase in babies being born, a reduction in the number of older persons and an increase in net migration, which impacts the younger adult age groups the most. This leads to a population with a younger age profile than the principal projection.

The other standard combination variants, high population and low population, were published with the 2014-based national population projections on 29 October 2015.

Special case scenarios

Replacement fertility projection

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. Replacement level is around 2.075 in the UK, that is, women would need to have, on average, 2.075 children each to ensure the long-term "natural" replacement of the population.

The replacement level is based on analysis published in the article Replacement fertility, what has it been and what does it mean? in Population Trends 119 in spring 2005. We recently recalculated the replacement fertility level but it was decided not to make a change since the impact on the resulting projections would be minimal. More information on this is available in the fertility discussion paper. (1.33 Mb Pdf)

The replacement fertility projection combines assumed replacement level fertility with the principal assumptions of mortality and migration.

In the 2014-based projections, UK replacement fertility rates are calculated additively, based on summing the births and population figures for the constituent countries. Replacement fertility rates for the UK may vary slightly from 2.075 due to rounding.

Constant fertility projection

This projection assumes that age specific fertility rates will remain constant at the values assumed for the first year (mid-2014 to mid-2015) of the principal projection. Although actual age-specific fertility rates for that year were not known when the principal projection was carried out, the assumed rates were consistent with provisional estimates of total births for the year. The constant fertility projection combines assumed constant level fertility with the principal assumptions of mortality and migration.

The total fertility rate in the constant fertility projection for the UK , at 1.81, is lower than the long-term assumption for the principal projection (1.89). Therefore, for this round of projections, projected population figures for the constant fertility variant are slightly lower than the principal projections.

In the 2014-based projections, UK constant fertility rates are calculated additively based on summing the births and population figures for the constituent countries. Constant fertility rates for the UK may vary slightly year on year due to rounding.

Table 1.1 shows the assumed long-term total fertility rates assumed for the standard variants compared with assumptions based on replacement and constant fertility.

No mortality improvement projection

This projection assumes that the combined age/sex specific mortality rates will remain constant at the values assumed for the first year (mid-2014 to mid-2015) of the principal projection. Although actual age/sex specific mortality rates for that year were not known when the principal projection was carried out, the assumed rates were 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.

In the 2014-based projections, "no mortality improvement" mortality rates for the UK are calculated additively based on summing the deaths and population figures for the constituent countries. Therefore "no mortality improvement" mortality rates and expectation of life figures for the UK may vary slightly year on year due to rounding.

Table 1.2 shows the effect on period expectation of life at birth in the year to mid-2039 with no mortality improvement compared to principal, high and low standard variants.

The no mortality improvement variant is different from the low life expectancy variant. The former assumes that mortality rates remain at the rates assumed from provisional data in the year ending mid-2015. The latter assumes that there is some improvement in mortality in the short-term, but that there is no improvement after 2039.

Zero net migration (natural change only) projection (published 29 October 2015)

This projection uses the principal assumptions of fertility and mortality and assumes that there will be zero net migration (for every age for each sex). It therefore shows the consequences of the principal assumptions of fertility and mortality in the absence of migration, or where migration inflows and outflows are exactly equal at every age.

When compared to the principal projection, the zero net migration (natural change only) projection allows the impact of the principal net migration assumption on the projected population to be assessed.

No change projection

This projection shows what would happen if fertility, mortality and net migration were to remain constant at current levels. It assumes the fertility rates from the constant fertility projection and the mortality rates from the no mortality improvement projection. However, given the fluctuating nature of net migration, it is much more difficult to define what is meant by the current level of net migration. So the principal migration assumptions have been used for the no change projection.

Long-term balanced net migration projection

The long-term balanced net migration variant assumes that net migration will decline to zero in the long-term (between 2022 and 2036), with in-migration and out-migration total flows being equal. However, although long-term total inflows and outflows are assumed to be equal, inflows and outflows are not necessarily assumed to be equal for each individual age or sex (so a net inflow at one age might be offset by a net outflow at another age). For the 2014-based projections the variant follows the assumptions of the low migration variant, as published 29 October 2015, in the run-in period to the long-term assumption (from the year ending mid-2015 to year ending mid-2021). In previous projections it followed the principal assumption. However, feedback at consultation showed users required the long-term balanced net migration to follow a lower level of migration. This variant is only produced for the UK.

Table 1.3 presents projections of the total population under the principal projection, single component variants, combination variants and special case scenarios for the UK in mid-2039 and compares this to the mid-2014 estimate.

Equivalent tables for the constituent countries and 3 additional measures, the percentage of the population under 16, percentage of population 65 and over and dependants per 1000 persons of working age (dependency ratios), are available in the data download.

Figure 1.1 illustrates how the total population is projected to change over the next 25 years for the principal, single component and combination variant projections.

The equivalent charts for the constituent countries of the UK can be found in Appendix B.

Figure 1.2 shows the estimated and projected percentage of the UK population aged 65 and over between mid-1981 and mid-2039 for the principal projection and selected standard variants.

All variants show an increase in the percentage of the total population in the UK aged 65 and over compared with the mid-2014 estimate (17.7%). The old population variant (assuming low fertility, high life expectancy and low net migration) projects the highest percentage across all variants for this age group with 26.3% of the UK population expected to be aged 65 and over by mid-2039.

The equivalent charts for the constituent countries of the UK can be found in Appendix C.

Figure 1.3 shows the estimated and projected percentage of the UK population aged under 16 between mid-1981 and mid-2039.

In contrast to the percentage of the population aged 65 and over, there is more consistency between the variants for the percentage of the population aged under 16. The young population variant (assuming high fertility, low life expectancy and high net migration) projects the highest percentage across all variants for this age group to be 19.4% in mid-2039, compared with 18.8% in mid-2014. In the long term, only the young population, high fertility, and high population variants project higher percentages of under 16s than at present.

The equivalent charts for the constituent countries of the UK can be found in Appendix D.