The true statistics for net migration could be 35,000 higher or lower than official estimates, Oxford University's Migration Observatory has said after the Office for National Statistics (ONS) released the provisional estimate for net migration in 2011 today (30 August).
The central estimate for net migration in 2011 was 216,000 – a fall of 36,000 from 2010's estimate of 252,000. But for the first time, ONS also released the margins for error associated with its net-migration statistics with the result that net migration could be 35,000 higher or lower than estimated.
The ONS notes that the 2011 decline in the estimate of net-migration is 'not statistically significant' – which means that it cannot be said for certain that net-migration has actually declined, despite the fall in the central estimate.
This is because the migration estimates are primarily based on a sample survey – the International Passenger Survey (IPS) – which, as is the case with all surveys, produces estimates that are subject to margins of error.
Dr Martin Ruhs, Director of the Migration Observatory at the University of Oxford, said: 'There is a constant desire among policy makers in all parties, the press and other interest groups in having 'hard' facts and specific numbers about migration, but the reality is that sometimes these are simply not available.
"The uncertainty around the official migration estimates means that the figures need to be used and interpreted with great care."
He added: "The uncertainty in the UK's migration estimates also means that it is very difficult to assess how well the government is progressing toward its target of reducing net-migration to the 'tens of thousands', or to evaluate the effects of specific policy changes."
"In simple terms, the Government could miss the "tens of thousands" target by many tens of thousands and still appear to have hit it – conversely the Government could hit, or even exceed its target and still appear to have missed it."
Explore further: Understanding the economics of human trafficking
More information: A comment piece by the Migration Observatory explores the importance and profound implications of this uncertainty for the UK's migration debate.