How the statistics are gathered, and why they do not add up

Passenger survey

The Government’s main set of data on immigration is the International Passenger Survey (IPS), based on interviews with thousands of travellers at ports and airports. From this, the ONS extrapolates an estimate of overall numbers of migrants arriving and leaving. It said 260,000 came from the EU last year. The accuracy of the main immigration figures has been challenged because it is an estimate based on this survey, not a headcount.

Short term vs long term

Because the IPS only counts long-term migrants coming for a year or more, those visiting for shorter periods will not be included as migrants. The ONS said that over the past five years 2.4 million EU migrants came to Britain when “short-term” migrants were added into the total. The main migration figure for the past five years only counted the 900,000 who said they were coming here long term.

National insurance data adds to confusion

The ONS also produced details of national insurance numbers allocated to foreign nationals in a bid to back up its official migration estimates. But these also appeared to undermine the main migration figures. EU migrants who used their national insurance numbers over “longer durations” – indicating they appeared to have remained in Britain rather than going home – were given as up to 1.1 million, even though official estimates said the figure was 739,000 over four years.