Figures out this morning will reveal how significant immigration and emigration has been in the past year – but how are the numbers gathered, and how reliable are they?
The system the UK uses to estimate long-term migration is now, in the words of the Office for National Statistics, “stretched beyond its purpose”.
The system’s critics are a little harsher. They say it offers at best an educated guess – and at worst is deeply flawed.
Every three months the Office for National Statistics (ONS) publishes a migration update. And at the heart of that report is the International Passenger Survey(IPS).
This enormous exercise was launched in 1961 to help the government better understand the impact of travel and tourism on the economy – but over the years, it became a rather useful way of estimating who was coming and going for broader political purposes.
When the David Cameron government set a net migration target of tens of thousands in 2010, the IPS projection became a very public measure of policy failure.
So how does the IPS come up with its figures, and why is it so controversial?
Limits to the Passenger Survey
For 362 days a year, IPS staff leap in front of travellers at 19 airports, eight ports and the Channel Tunnel rail link. They ask them for a bit of their life story: where they’re from, why they’re in the UK and how long they might be staying.
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Up to 800,000 people a year take part and about 250,000 of those results are put through the statistical mixer to come up with an estimate for the number of people either arriving to live in the UK, or leaving the UK for at least a year – the internationally-agreed definition of a long-term migrant.
But the IPS doesn’t cover all the ports, all of the time. Take Dover, for example. There are up to 51 ferry arrivals a day – and the IPS only covers four days’ worth of them over an entire year. The nationwide sample works out at just over 1% of Heathrow’s annual traffic of 78 million people.
Within the sample, the number of known migrants is about 5,000. So they are an incredibly small group, within an already small sample, upon which statisticians are expected to give ministers some big answers.