Many will be familiar with the idea of ‘net migration targets’ and bids to ‘reduce net migration to the tens of thousands’. But fewer people know that the UK’s official net migration figures are estimates based primarily on an annual survey of around 3,000 people and come with a large margin of uncertainty attached. In this Insight – the latest in our series on migration stats – we look at where these figures come from and what you should know when interpreting them.
How is net migration calculated?
Immigration – Emigration = Net migration. In other words, net migration is the number of people migrating to the UK minus the number migrating out of it. Net migration in the tens of thousands means that: Immigration – Emigration < 100,000
The UK’s official net migration figures are estimates based on a continuous survey of people travelling through UK air, rail, and sea ports. Originally designed to simply collect information on passengers’ journeys, this survey is now our primary means of estimating the flow of migrants in and out of the country.
Migrants: needles in a haystack
There were around 137 million passenger arrivals to the UK in 2017, of which around 630,000 were long-term international immigrants (less than 0.5% of all arrivals). 
The International Passenger Survey (IPS) is a rolling survey of passengers entering and leaving the UK. It takes a sample of people passing across the UK border and uses the information collected on them to estimate the UK’s official immigration, emigration, and net migration figures.
The IPS, which began in 1961, was originally designed to capture information about tourism and business travel. It takes a sample of all passengers passing through, so it picks up far more tourists than long-term migrants.
In fact, the survey samples ‘around 800,000’ passengers per year, of which around 3,000 (0.4%) are identified as long-term migrants. The map below shows the number of migrants interviewed by location in 2016.
Someone responding to the IPS is counted as a ‘long-term immigrant’ if they are intending to stay in the UK for one year or more and vice versa for long-term emigrants.
Their answers to questions about where they are going, their reason for coming to the UK, and how long they are intending to stay are also used to make estimates about all migrants.
How precise are net migration estimates?
In 2017, net migration was estimated at 282,000 with statisticians 95% confident that the estimate is correct -/+47,000. This means that if the assumptions on which the data are collected and analysed hold, it is highly likely that the ‘true’ net figure is between 235,000 and 329,000.
The chart below shows this margin of uncertainty around the annual main estimates. Since there is a sufficient gap between immigration and emigration, we can at least be certain that net migration is positive (more immigration than emigration).
Note that the Census in 2011 allowed us to more precisely estimate the increase in the population that was due to migration for each of the previous ten years, so there is no ‘margin of error’ between 2001 and 2011.
These estimates come with uncertainty because the survey sample (3,000 people) is small compared to the population of migrants (630,000 arriving and 350,000 leaving).
The number of migrants has risen over time, but the sample size has stayed roughly the same. As the next chart shows, this means that one migrant responding to the survey now ‘represents’ far more people than ever before.
As well as sampling issues, there are other reasons for uncertainty in the estimates:
- As an intentions-based survey, there is always a risk that what people say they intend to do will not happen.
- There are some gaps in the survey’s coverage, for example that it is not conducted at all airports, which the ONS must fill in based on assumptions.
- No one is interviewed between the hours of 10pm and 6am, yet 15% of flights arrive during this time.
- The survey is also not conducted along the UK-Ireland land border so the ONS adds in data from the Irish Central Statistical Office to estimate flows to and from Ireland.
The ONS also adds in the number of asylum seekers and refugees (who usually do not pass through the arrivals terminal) and uses Home Office data to estimate how many people extend or switch their visa once already in the UK.
Stretched beyond its purpose
The IPS has received heavy criticism but the ONS still considers it “the best source of information to measure long-term international migration” at the national level.
A 2013 report by the Public Administration Select Committee called it, “little better than a best guess” and the chair claimed that:
“If you try to work out, say, how many Egyptians or Syrians came to the UK last year, any numbers are virtually meaningless.”
It is true that because the sample size is so small, IPS estimates of migration by nationality and migration to local areas are very imprecise. The confidence interval around an estimate can be as large as the estimate itself and the ONS deems these small area estimates ‘not reliable’.
The main alternative to measuring migration flows using a passenger survey is to have a centralised population register. Such systems are also vulnerable to inaccuracy, as it is virtually impossible to enforce any penalties for people failing to de-register if they leave the country.
The ONS acknowledges that the IPS has been “stretched beyond its purpose” and is currently working on ways of combining its data with Home Office data on visas and border checks.
For a comprehensive guide to understanding migration statistics read Migration Statistics, our guide to the concepts and methods used in measuring migration. For more detail on the IPS and its limitations read the BBC’s Reality Check Do we really know the scale of UK migration?
 Table ad_01 https://www.gov.uk/government/publications/immigration-statistics-year-ending-march-2018/list-of-tables
Georgina Sturge is a Statistical Researcher specialising in social and general statistics at the House of Commons Library.