People are very hard to count, especially in a free society. The failings in Britain's system of counting migration reflect the inherent flaws in any mass sampling system. Although the system could be improved, it will always be tough to predict future trendsby Michael Blastland / November 25, 2007 / Leave a comment
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In late September, the Office for National Statistics (ONS) sharply increased its projection for annual net immigration to Britain—from 145,000 to 190,000. Few numbers light the political touchpaper quite like these. The increase is based partly on data for two new years (2004 and 2005), when net migration was at record levels, and partly on methodological changes. In practical terms, this means in 2005-06 an estimated net inflow of about 500 people a day; in future, the ONS assumes, a few more.
Since scepticism about immigration often expresses itself as cynicism about the statistics, it is worth looking more closely at where the numbers come from. A common assumption is that we count immigrants, or try, and that if we get it wrong, it is because some slip under the wire, through illegal channels, official incompetence or conspiracy. The truth is that we don’t even do that. Few statistics about the economy and society are the result of a count. The migration number comes from a sample, and it is a good illustration of sampling’s limitations. Some people, hearing how it is done, don’t quite believe it.
The international passenger survey
The story begins with the International Passenger Survey (IPS). The IPS is a sample of all people entering and leaving the country—including migrants, who are defined as those intending to settle or leave for a year or more. The IPS interviews about 300,000 people a year on boats, at the channel tunnel and, mostly (about 86 per cent), at airports.
You may have seen them in action on a sea crossing. In matching blue blazers, a survey team finds its ferry, stands at the top of the various stairs to the passenger decks and selects a sample by scribbling a description of every tenth person aboard: the rucksacked, the refugees, the suited or the carefree. The plan is to use this description to pick them out later, during the voyage, for a gentle interrogation, hoping for no strange languages; hoping no one’s in the shower, asleep under the newspaper, or in a change of clothes; hoping they will be willing to answer questions. For the only way to know whether they are migrants rather than tourists, on business or a booze cruise, is to ask them. This gives the origin…