That's the question millions will be asking when they see the new paper by my friends at the University of Massachusetts, Thomas Herndon, Michael Ash, and Robert Pollin. Herndon, Ash, and Pollin (HAP) corrected the spreadsheets of Carmen Reinhart and Ken Rogoff. They show the correct numbers tell a very different story about the relationship between debt and GDP growth than the one that Reinhart and Rogoff have been hawking.

Just to remind folks, Reinhart and Rogoff (R&R) are the authors of the widely acclaimed book on the history of financial crises, This Time is Different. They have also done several papers derived from this research, the main conclusion of which is that high ratios of debt to GDP lead to a long periods of slow growth. Their storyline is that 90% is a cut-off line, with countries with debt-to-GDP ratios above this level seeing markedly slower growth than countries that have debt-to-GDP ratios below this level. The moral is to make sure the debt-to-GDP ratio does not get above 90%.

There are all sorts of good reasons for questioning this logic. First, there is good reason for believing causation goes the other way. Countries are likely to have high debt-to-GDP ratios because they are having serious economic problems.

Second, as Josh Bivens and John Irons have pointed out, the story of the bad growth in high debt years in the United States is driven by the demobilization after the second world war. In other words, these were not bad economic times; the years of high debt in the United States had slow growth because millions of women opted to leave the paid labor force.

Third, the whole notion of public debt turns out to be ill-defined. Countries can sell off assets to pay down debts: would this avoid the R&R high debt twilight zone of slow growth? In fact, even the value of debt itself is not constant. Long-term debt issued in times of low interest rates will fall in value when interest rates rise. If there is a high debt twilight zone effect as R&R claim, then we can just buy back bonds at steep discounts and send our debt-to-GDP ratio plummeting.

But HAP tells us that we need not concern ourselves with any arguments this complicated. The basic R&R story was simply the result of them getting their own numbers wrong.

After being unable to reproduce R&R's results with publicly available data, HAP were able to get the spreadsheets (zip) that R&R had used for their calculations. It turns out that the initial results were driven by simple computational and transcription errors. The most important of these errors was excluding four years of growth data from New Zealand in which it was above the 90% debt-to-GDP threshold. When these four years are added in, the average growth rate in New Zealand for its high debt years was 2.6%, compared to the -7.6% that R&R had entered in their calculation.

Since R&R country-weight their data (each country's growth rate has the same weight), and there are only seven countries that cross into the high-debt region, correcting this one mistake alone adds 1.5 percentage points to the average growth rate for the high-debt countries. This eliminates most of the falloff in growth that R&R find from high debt levels. (HAP find several other important errors in the R&R paper, but the missing New Zealand years are the biggest part of the story.)

This is a big deal because politicians around the world have used this finding from R&R to justify austerity measures that have slowed growth and raised unemployment.

In the United States, many politicians have pointed to R&R's work as justification for deficit reduction even though the economy is far below full employment by any reasonable measure. In Europe, R&R's work and its derivatives have been used to justify austerity policies that have pushed the unemployment rate over 10% for the eurozone as a whole and above 20% in Greece and Spain. In other words, this is a mistake that has had enormous consequences.

In fairness, there has been other research that makes similar claims, including more recent work by Reinhardt and Rogoff. But it was the initial R&R papers that created the framework for most of the subsequent policy debate. And HAP has shown that the key finding that debt slows growth was driven overwhelmingly by the exclusion of four years of data from New Zealand.

If facts mattered in economic policy debates, this should be the cause for a major reassessment of the deficit reduction policies being pursued in the United States and elsewhere. It should also cause reporters to be a bit slower to accept such sweeping claims at face value.

• This article was first published on 16 April on the CEPR blog and is crossposted by the author's permission