Migration statistics are widely cited and used as the basis for a range of policy decisions. But did you know that most migration statistics do not come from an actual count of people but are estimates based on surveys? A new series from the House of Commons Library explains the origins of migration statistics and what you should be aware of when interpreting them.

It might be assumed that we know how many migrants live in the UK. It may also seem safe to assume that we should know the size of the UK population or the population of a local area, at any given point in time.

The fact is, we know neither of these for certain and our official statistics of both are carefully calculated estimates.

The UK does not maintain a central list of everyone who is resident in the country at a given time. Only in Census years (usually every tenth year), when the whole population is counted and their nationality or country of birth recorded, do we know how many people live where and which countries they migrated from. In between Census years, the Office for National Statistics (ONS) makes official estimates of the number of migrants in the population.

Who is a ‘migrant’?

In statistical terms, a migrant is someone who changes their place of usual residence. An immigrant is someone who has come to the UK, as opposed to emigrants who have left the UK. What sets migrants apart from the rest of the population is that they either:

  1. Were born in a different country or
  2. Have the nationality of a different country.

However, the ONS uses the UN’s definition of a migrant as a ‘long-term’ resident, so migrants to the UK are people who come to live for 12 months or more. This 12-month residence definition is widely used, for example by the EU statistics agency, Eurostat.

Neither of these definitions perfectly reflect the complexity of how people define themselves. People who were born abroad to British parents and automatically acquired British nationality would be classified as migrants under the ‘country of birth’ definition. Nationality can change, for example if someone lives in a country long enough to apply for citizenship, which means that some people identified as migrants in the data may have since become British citizens.

Census years provide a snapshot

The Census is a survey of the entire population so, in theory, it can tell us exactly how many migrants live in the UK. In reality, of course, some people will be missed from the Census for various reasons, but for statisticians, this is about as precise as data on people can get.

At the 2011 Census, there were 63.2 million people living in the UK, of whom 8 million were born abroad.

The Census did not ask about people’s nationality but about their country of birth and the passports they held. In 2011, 17% of the population of England and Wales did not have a passport, so this is not a perfect indicator of the proportion who are migrants.

In between censuses, we rely on survey data

The ONS produces annual estimates of the UK’s population by country of birth and nationality which is based on data from the Annual Population Survey (APS).

The APS, which began in 2004, collects data on around 112,000 households containing 320,000 individuals per year.

Around 80% of the sample comes from the UK Labour Force Survey (LFS) and the remainder is from ‘sample boosts’, which survey additional people in areas with small populations. The idea behind the boosts is to pick up sub-groups within the population, such as migrants.

The latest ONS estimate, for 2017, is that there were 9.4 million foreign-born people in the UK (14% of the population) and 6.2 million non-UK nationals (10%) of the population.

Small area estimates are not very precise

The ONS also makes estimates about the migrant population at the country, region, and local authority level. Since these are based on a small sample of people living in each area, the estimates are sometimes very imprecise when it comes to estimating the number of migrants.

The main estimate of foreign nationals living in each local area in 2017 is positive in all cases but one: Rutland was estimated to have no foreign national residents.

However, the large margin of uncertainty around these estimates means that in one in five local authorities, we cannot be sure that the number of foreign nationals is not zero. In other words, the official estimate includes the possibility that there is not a single foreign national living in these places.

The map below illustrates the uncertainty of some of the 2017 estimates.

Migration in the UK estimates

Areas with very few migrants tend to have a high level of statistical uncertainty, in the sense that the true number of migrants could be double or triple the main estimate, or it could be zero.

In areas with a large population, the estimate can come with a ‘confidence interval’ of plus or minus 10-20,000: for example, Birmingham’s migrant population is estimated at 144,000 but it could be 118,000 (11% of Birmingham’s population) or it could be 170,000 (15% of its population).

It is important to take these small area estimates with a pinch of salt. The ONS itself cautions against using an estimate if the confidence interval is higher than the estimate itself. As the map illustrates, this means that the APS does not really provide us with usable data on the migrant population for about one in five local areas. At the regional and national levels the estimates are much more reliable.

Related reading:

Migration Statistics, House of Commons Library.

Georgina Sturge is a Statistical Researcher specialising in social and general statistics at the House of Commons Library.