Chart of the week: Even the past is murky to economists

Not only can economists not predict the future, but they can't even describe the past, says Roger Bootle of Capital Economics. Measuring GDP, a country's economic output, is an inexact and ongoing science.

The Office for National Statistics has revised data going back to 1998. The upshot is that the 2000-2007 boom was not quite as strong as believed at the time, while the recovery from the slump was stronger.

GDP in 2009 was 0.9% stronger than initially estimated, for instance, and the double-dip recession of 2012 has been revised away.The economy expanded by 0.1% at the start of 2012, compared to a first estimate that pointed to a 0.2% decline.

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The overall slide from the pre-crisis peak in early 2008 to the trough the following year has been upgraded from 7.1% to 6%.

The numbers may well be tweaked further in future, but for now this means output reached its pre-crisis level in the middle of 2013 a year earlier than we thought.