Danny Finkelstein says we should forget the figures on public finances. This is especially true of the forecasts for them. My table shows the recent track record for Pre-Budget forecasts of public sector net borrowing in the following year, compared to the out-turn (table PSF7 of this pdf and the consensus forecast here (pdf) for 2007-08).
The forecast errors are large - the average, ignoring signs, is £7.6bn, or roughly 0.6% of this year's likely annual GDP. This is more than twice the shortfall which Darling alleges there to be in Osborne's tax plans.
Although these errors seem big, they are in fact smaller than previous ones - thanks in part to the macroeconomic stability Darling will tell us about this afternoon. Back in 1998, the Treasury estimated that the forecast error was equivalent to 1 per cent of GDP.
Of course, you won't hear about these forecast errors from Darling. But when he gives his new forecasts, just add the phrase "give or take £8 billion" to what he says.
This should put in context the parties' quibbles about a couple of billions here or there in their opponents' sums.