Politicians’ memoirs are ten-a-penny while books by statisticians in the House of Commons Library can be counted on one thumb. This is that book and its rarity makes it all the more valuable. We are familiar with the flood of Government statistics; what is less apparent to the reader is how the data behind the statistics was collected. This book exposes how unreliable such data can be and how it can mislead even well-intentioned politicians.

Sturge provides a number of examples. We remember Gordon Brown meeting Gillian Duffy in Rochdale during the 2010 General Election, but what Brown and other politicians did not appreciate at the time was the level of immigration to the UK from the A8 countries. This was because for decades immigration had been estimated using the Air Passenger Transport Survey which sampled travellers passing through Heathrow, Gatwick and Manchester airports, and this sampling had led to  estimates close enough to the decadal censuses that there was no good reason to change the sampling process.

What happened in the 2000s was that a Hungarian businessman Jóseph Váradi co-founded a low-cost airline, Wizz Air, which like other low-cost airlines, flew to small regional airports. The UK Government in anticipation of the A8 countries joining the EU had asked the statisticians for an estimate of the number of migrants from these countries coming to the UK and received a response of 5 to 13 thousand per year.

That had been based on an assumption that between 20 and 73 thousand per year would emigrate to Germany, but just before the enlargement the German Government had paused immigration from the A8 countries for two years. Not surprisingly many Eastern Europeans, particularly Poles, chose to come to Britain instead.

Unlike Germany, where any migrant has to register at their local Citizens Office within 14 days to live and work legally, there is no single action that a migrant needs to do in the UK and no link between National Insurance numbers and NHS numbers, nor are these linked to council records. As a result there is no easy way to make an estimate of immigration between censuses.

Another example Sturge uses from the Blair years is the change in agricultural subsidies from production to farmed area. Although land is registered on change of ownership, there were large areas of unregistered UK land: think of the land owned by the Crown, the Church, and Oxbridge Colleges. One consequence of the change is that it created an incentive to register land, even if it wasn’t actively farmed. In 2005 there were 100,000 applications to register land up from 9,000 previously.

And errors like this are not confined to the 21st Century. Back at the beginning of the 20th Century when the Liberal Government introduced the first old age pension on 1st January 1909, they did not have accurate birth records, because the requirement to register all births only became compulsory in 1875. The cost was predicted to be £1.2 million in the first year, but actually cost £2.1 million, rising to £9.7 million in 1910 and nearly £13 million in 1911.

What all these stories, and many others in the book, have in common is the need to examine closely just how the data for the statistics was collected. If it causes politicians and electors to treat Government statistics with a sceptical eye and demand information on how the data was collected, not just accept the figures, it will have served its purpose.

Bad Data is published by Bridge Street Press £20 (hardback) ISBN 978-0-349-12861-0

* Laurence Cox has been a party member and activist since 1981 and is currently secretary of his local party.