By PAUL KRUGMAN
Published: April 18, 2013
※ This reader's apology: 'copy and paste' of somebody else's document is a bad thing. But I cannot help doing it here just for a short time onlyㅡmaybe 2-3 days to one weekㅡbecause in this column of Paul Krugman there are many curious hyperlink sources, and I want to keep them for a bit later reference. I will remove the main body of the article copied here after reading and checking the content and keeping some sources.
( ... ) NASA’s Mars Orbiter crashed because engineers forgot to convert to metric measurements; JPMorgan Chase’s “London Whale” venture went bad in part because modelers divided by a sum instead of an average. So, did an Excel coding error destroy the economies of the Western world?
The story so far: At the beginning of 2010, two Harvard economists, Carmen Reinhart and Kenneth Rogoff, circulated a paper, “Growth in a Time of Debt,” that purported to identify a critical “threshold,” a tipping point, for government indebtedness. Once debt exceeds 90 percent of gross domestic product, they claimed, economic growth drops off sharply.
Ms. Reinhart and Mr. Rogoff had credibility thanks to a widely admired earlier book on the history of financial crises, and their timing was impeccable. The paper came out just after Greece went into crisis and played right into the desire of many officials to “pivot” from stimulus to austerity. As a result, the paper instantly became famous; it was, and is, surely the most influential economic analysis of recent years.
( ... ) their tipping-point claim was treated not as a disputed hypothesis but as unquestioned fact. For example, a Washington Post editorial earlier this year warned against any relaxation on the deficit front, ( ... )
For the truth is that Reinhart-Rogoff faced substantial criticism from the start, and the controversy grew over time. As soon as the paper was released, many economists pointed out that a negative correlation between debt and economic performance need not mean that high debt causes low growth. It could just as easily be the other way around, with poor economic performance leading to high debt. Indeed, that’s obviously the case for Japan, which went deep into debt only after its growth collapsed in the early 1990s.
Over time, another problem emerged: Other researchers, using seemingly comparable data on debt and growth, couldn’t replicate the Reinhart-Rogoff results. They typically found some correlation between high debt and slow growth — but nothing that looked like a tipping point at 90 percent or, indeed, any particular level of debt.
Finally, Ms. Reinhart and Mr. Rogoff allowed researchers at the University of Massachusetts to look at their original spreadsheet — and the mystery of the irreproducible results was solved. First, they omitted some data; second, they used unusual and highly questionable statistical procedures; and finally, yes, they made an Excel coding error. Correct these oddities and errors, and you get what other researchers have found: some correlation between high debt and slow growth, with no indication of which is causing which, but no sign at all of that 90 percent “threshold.”
In response, Ms. Reinhart and Mr. Rogoff have acknowledged the coding error, defended their other decisions and claimed that they never asserted that debt necessarily causes slow growth. That’s a bit disingenuous because they repeatedly insinuated that proposition even if they avoided saying it outright. But, in any case, what really matters isn’t what they meant to say, it’s how their work was read: Austerity enthusiasts trumpeted that supposed 90 percent tipping point as a proven fact and a reason to slash government spending even in the face of mass unemployment.
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