The most important paragraph in the IMF World Economic Outlook

68 words of wonkishness.

The IMF's World Economic Outlook (pdf) – a 230-page tome detailing predictions on nearly every aspect of the world's economy collated by the international organisation – always gets attention for the calls it makes.

The October edition downgrades expected global growth for 2013 from 3.9 per cent to 3.6 per cent, and also cuts predictions for China (down to 8.2 per cent for 2013), the US (expected to grow by 2.1 per cent in 2013, down from 2.3 per cent in July's prediction) and the UK (now expected to grow by just 1.1 per cent next year, and to contract by 0.2 per cent this year).

But the predictions are not the most important passages in this edition of the Outlook. Those are found in a short box-out titled Are We Underestimating Short-Term Fiscal Multipliers?

The fisc§al multiplier is the effect government spending has on GDP. Money spent by the government doesn't disappear – it is respent, again and again. If a teacher gets a pay rise, their consumption is likely to rise in line with it; if all teachers get pay rises, that increase in consumption may be enough to affect the aggregate demand in the economy. In an economy which isn't being stretched to its limits – that is, one without full employment, or serious capital equipment shortages – that increase in aggregate demand will result in an increase in GDP.

The existence of the fiscal multiplier is a matter of fact, but the magnitude of it is contested. And that's where the IMF enters the scene, on page 42:

The main finding, based on data for 28 economies, is that the multipliers used in generating growth forecasts have been systematically too low since the start of the Great Recession, by 0.4 to 1.2, depending on the forecast source and the specifics of the estimation approach. Informal evidence suggests that the multipliers implicitly used to generate these forecasts are about 0.5. So actual multipliers may be higher, in the range of 0.9 to 1.7.

Emphasis mine. When deciding how much to spend, governments have been assuming that every pound they spend increases GDP by 50p – but it may increase it by as much as £1.70.

The reason this is so very important is that fiscal multiplier is usually appealed to not when deciding how much to spend, but how much to not spend. When governments are planning austerity packages, they have to be wary of the fact that large cuts to government spending will inevitably cause a decrease in output, and so they either have to be prepared to take that hit, or come up with a reason why slashing spending will cause an increase in output through some other mechanism.

That is easy enough to do if you are trying to account for a fiscal multiplier of 0.5: you can make the arguments, which Osborne and Cameron rehearsed repeatedly, that the public sector is crowding out the private; that the government spending which is being cut is particularly inefficient; or that the confidence fairies will reward your thriftiness with growth.

When there is the chance that the fiscal multiplier is three times that, austerity becomes much more likely to involve damaging drops in output.

There was once a time when the government pegged its credibility to that of the IMF – back when George Osborne was proud about Britain's credit ratings, and the international community was behind his plans. Those days are gone, and have been since Christine Lagarde made her own attack on austerity. But the economists at the treasury may be more inclined to listen to the wonkish findings of the World Outlook than the political interventions of the fund's leader. We can only hope they are prompted to re-do the sums.

The IMF headquarters. Photograph: Getty Images

Alex Hern is a technology reporter for the Guardian. He was formerly staff writer at the New Statesman. You should follow Alex on Twitter.

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To stop Jeremy Corbyn, I am giving my second preference to Andy Burnham

The big question is whether Andy Burnham or Yvette Cooper will face Jeremy in the final round of this election.

Voting is now underway in the Labour leadership election. There can be no doubt that Jeremy Corbyn is the frontrunner, but the race isn't over yet.

I know from conversations across the country that many voters still haven't made up their mind.

Some are drawn to Jeremy's promises of a new Jerusalem and endless spending, but worried that these endless promises, with no credibility, will only serve to lose us the next general election.

Others are certain that a Jeremy victory is really a win for Cameron and Osborne, but don't know who is the best alternative to vote for.

I am supporting Liz Kendall and will give her my first preference. But polling data is brutally clear: the big question is whether Andy Burnham or Yvette Cooper will face Jeremy in the final round of this election.

Andy can win. He can draw together support from across the party, motivated by his history of loyalty to the Labour movement, his passionate appeal for unity in fighting the Tories, and the findings of every poll of the general public in this campaign that he is best placed candidate to win the next general election.

Yvette, in contrast, would lose to Jeremy Corbyn and lose heavily. Evidence from data collected by all the campaigns – except (apparently) Yvette's own – shows this. All publicly available polling shows the same. If Andy drops out of the race, a large part of the broad coalition he attracts will vote for Jeremy. If Yvette is knocked out, her support firmly swings behind Andy.

We will all have our views about the different candidates, but the real choice for our country is between a Labour government and the ongoing rightwing agenda of the Tories.

I am in politics to make a real difference to the lives of my constituents. We are all in the Labour movement to get behind the beliefs that unite all in our party.

In the crucial choice we are making right now, I have no doubt that a vote for Jeremy would be the wrong choice – throwing away the next election, and with it hope for the next decade.

A vote for Yvette gets the same result – her defeat by Jeremy, and Jeremy's defeat to Cameron and Osborne.

In the crucial choice between Yvette and Andy, Andy will get my second preference so we can have the best hope of keeping the fight for our party alive, and the best hope for the future of our country too.

Tom Blenkinsop is the Labour MP for Middlesbrough South and East Cleveland