Migration assumptions

This report provides detailed information on the principal and variant migration assumptions used in the 2014-based national population projections. The new long-term assumption for net migration to the UK is +185,000 each year, compared with +165,000 per year in the 2012-based projections.

All 2014-based figures in this report are rounded to the nearest hundred.

Revised methodology

Following a methodology review (456.7 Kb Pdf) carried out by the Economic and Social Research Council Centre for Population Change in 2012, the international migration assumptions methodology was changed in the 2012-based projections. Another recommendation was the use of migration rates for certain migrant flows. 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 (399 Kb Pdf) on our website.

International migration

Assumptions of future international migration have been derived from modelling recent trends in civilian migration to and from the UK. Migrants are defined as individuals who change their country of usual residence for a period of at least a year, so that the country of destination becomes the country of usual residence.

International migration figures are derived from a number of sources. The principal source is the International Passenger Survey (IPS). Adjustments are made to account for people who enter or leave the country initially for a short stay but subsequently decide to remain for a year or more ("visitor switchers") and people who originally intend to be migrants but in reality stay in the UK or abroad for less than 1 year ("migrant switchers"). Flows to and from the Republic of Ireland, taking into account the discontinuity in 2008 due to methodological changes, are included in the IPS flows.

The IPS also excludes most, but not all, people seeking asylum. Estimates of the flows of asylum seekers (and their dependants) not captured by the IPS are obtained from Home Office data.

Cross-border migration

A new improved method for calculating cross-border migration assumptions is being implemented for the 2014-based national population projections. The assumptions for the flows between the countries of the UK are now set as rates which are based on National Health Service Central Register (NHSCR) trend data from the previous 5 years.

Annual age and sex-specific migration rates for each cross-border flow are calculated as the number of migrants at the end of the year divided by population of the country of origin at the start of the year. An average of the rates for the last 5 years of actual data (year ending mid-2010 to year ending mid-2014) is then taken and applied to the population of the country of origin at the beginning of each projection year to calculate the projected number of migrants for each flow. The main advantage of applying rates for cross-border migration is that the migrant flows are linked to the changing underlying population size and age structure. This means that the projections cannot produce implausible values, such as negative population stocks, when projected fixed levels of emigration are greater than the initial population size.

An adjustment has also been applied to the rates to take the population of the country of destination into account, ensuring that net migration levels between countries of the UK are stabilised over the course of the projection. More detail can be found in the cross-border methodology (399 Kb Pdf) document on our website.

Northern Ireland

From 2008, ONS migration estimates no longer use IPS data for Northern Ireland and instead use data from the Northern Ireland Statistics and Research Agency (NISRA). In order to obtain the longest possible continuous time series, Northern Ireland data back to 1992 are therefore obtained directly from NISRA.

Long-term assumptions

The long-term net international migration assumptions in the 2014-based projections take effect from the year ending mid-2021.

The new long-term assumption for net international migration to the UK is +185,000 each year compared with +165,000 a year in the previous projections. This increase in net migration results from recent changes in the UK migration figures. The annual net international migration figure of +185,000 is higher than the expectations of the expert advisory panel (+153,000). The difference is due predominantly to the emigration estimate which experts on average predicted to be 383,000 per annum, compared with a proposed assumption of 333,500.

The breakdown of the long-term net international migration assumption between the 4 countries within the UK is shown in Table 5.1. This table does not include the assumptions for cross-border flows between the constituent countries of the UK since these vary over the course of the projection due to the new rates-based method being used. The cross-border flows are considered separately.

Short-term assumptions

Special assumptions have been applied for the first few years of the projections (mid-2015 to mid-2020). These assumptions, shown in Table 5.2, have been formulated to represent a transition from the last year of actual data to the constant long-term assumptions. They also take into account the following factors:

1. further information on migration from the Migration Statistics Quarterly Report, published in August 2015

2. a short-term armed forces flow, which has been included to account for the planned return of home armed forces personnel and their dependents from Germany

Age and sex distributions

Figure 5.2 shows assumed long-term annual net international migration by age and sex for the UK from the year ending mid-2021 onwards. The international distributions are derived from an average of 5 years’ unsmoothed population estimates components of change data.

The assumptions for the flows between the countries of the UK are now set as rates instead of fixed numbers of migrants. Annual age and sex-specific migration rates for each cross-border flow are calculated as the number of migrants at the end of the year divided by population of the country of origin at the start of the year. An average of the rates for the last 5 years of actual data (year ending mid-2010 to year ending mid-2014) is then taken and applied to the population of the country of origin at the beginning of each projection year to calculate the projected number of migrants for each flow.

Figure 5.3 shows the trend in assumed net cross-border migration to England and how it compares with the 2012-based projections. The 2014-based assumptions show fluctuations in the earlier years but then stabilise, while the 2012-based assumptions were set as fixed numbers of migrants. The fluctuations occurring in the earlier years are due to the new method taking into account the underlying age and sex structure and population size of the countries.

The underlying annual age and sex-specific migration rates (adjusted and unadjusted) and the resulting number of migrants for each cross-border flow are available to download in the reference tables.

The underlying age and sex distributions for cross-border migration are based on data from NISRA, NRS and ONS.

The equivalent figures for Wales, Scotland and Northern Ireland are given in Figure 5.4, Figure 5.5 and Figure 5.6.

The standard high and low migration variants are produced by varying the international in- and out- flow assumptions and using the principal assumptions for all other flows.

Table 5.3 shows the assumed long-term annual net migration for the standard variants for the UK and its constituent countries. In addition to that, migration variants have also been produced for Great Britain for the first time and can be found in the published reference tables.

In the standard variants, the long-term levels are assumed from the year ending mid-2021 onwards. The 2014-based projections include high and low migration variants that are "additive" for the first time. This means the UK assumptions are equal to the sum of the 4 individual countries.

In the special case scenario, the projection uses the principal assumptions for fertility and mortality and assumes that there will be zero net migration for every age for each sex. When compared to the principal projection, the zero net migration projection allows the impact of the principal net migration assumption on the projected population to be assessed.

Figure 5.7 shows estimated and assumed net international migration to the UK between year ending mid-1994 and year ending mid-2025 for the principal projection and the high and low variants.

The equivalent figures for the constituent countries of the UK can be found in the relevant chart section of the appendices.