Britain’s net migration figure is surprisingly dodgy

IF INSANITY is doing the same thing over and over again and expecting different results, British politicians’ repeated promises on immigration might well be judged to be mad. In 2010 David Cameron vowed to reduce net migration (immigration minus emigration) to the tens of thousands. Theresa May, his home secretary and now successor as prime minister, has intimated that she will stick to this target in her Tory party’s manifesto for the forthcoming election. So far the promise has proved impossible to keep. Last year net migration was 273,000, even higher than in 2010.

Debate over immigration and how best to control it has become no less feverish since Britain’s vote to leave the European Union. Yet given how politicised the net migration figure has become, the data behind it are surprisingly unreliable.

The statistic is based on the International Passenger Survey. The IPS dates back to 1961, when the government established a travel and tourism survey to gather information about the movement of people. It was not designed to be the basis for measuring immigration but has since become so because Britain does not ask migrants to register after their arrival.

Between 6am and 10pm officials at ports and airports ask travellers questions about themselves, their destination and the purpose of their entry. Between 700,000 and 800,000 interviews are conducted a year (in 2015 foreigners made more than 36m trips to Britain). The data for the net migration figures are extrapolated from 4,000-5,000 interviews of people identified as long-term international migrants. The UK Statistics Authority admits that the margins of uncertainty are wide.

Britain is rare in using this kind of survey to count migrants, says Carlos Vargas-Silva of the Migration Observatory at the University of Oxford. In the EU, Cyprus is the only other country to do so. Countries such as America use household surveys. In 2015 Britain introduced a form of exit controls using information provided by airlines. But since it is not connected to the Home Office’s data on visas, it is no help in calculating levels of immigration, points out Mr Vargas-Silva.

Recently debate has focused on whether to remove foreign students from the overall figure, something Mrs May is apparently resisting. Those in favour argue that since overseas students stay only temporarily, are of little concern to the public (even those who want to reduce immigration substantially are relaxed about students, surveys show) and bring particularly clear economic benefits, they should be treated separately from other migrants.

That is all true, but taking students out of the migration calculations might not change the net figure much. It is increasingly difficult for them to stay in Britain after their studies; the government abolished post-study work visas in 2012. There is little evidence that many stay on illegally. That suggests the inflow and outflow should roughly balance out, meaning there would be little impact on the net migration figure if they were removed from it.

The IPS is particularly inaccurate when looking at subsets of migrants, such as students, since the sample size is even smaller. Alternatives to the current measure, such as household surveys, would be more reliable. Determining more accurately how many foreigners arrive in and leave Britain might be a good start to deciding immigration policy—and maybe even coming up with a more sensible one.

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