Sunday, May 26, 2013

Austerity economics only works if you make an Excel formula error

from BoingBoing:
A new paper called Does High Public Debt Consistently Stifle Economic Growth? A Critique of Reinhart and Rogoff by Thomas Herndon, Michael Ash, and Robert Pollin from UMass Amherst tries and fails to replicate the classic work on austerity, Carmen Reinhart and Kenneth Rogoff's 2010 Growth in a Time of Debt.

Reinhart-Rogoff is the main research cited in favor of cutting public services and spending in bad economic times. It's a big part of why the local library is shutting down, why they're kicking people out of public housing, shutting down arts programs, slashing education and public transit, and laying off public employees. It purports to show that countries with high debt-to-GDP ratios of 90 percent or more are a "threat to sustainable economic growth."

In the new Amherst paper, the authors reexamine Reinhart-Rogoff's original data and conclude that the numbers don't add up. They show that Reinhart-Rogoff cherry-picked which years of high-debt GDP they measure, that they put their thumbs on the scales with "unconventional weighting" and made a "coding error" that "entirely excludes five countries, Australia, Austria, Belgium, Canada, and Denmark." This last error -- literally the wrong formula in a spreadsheet cell -- badly skews the outcome.

Here's the tl;dr: "the average real GDP growth rate for countries carrying a public debt-to-GDP ratio of over 90 percent is actually 2.2 percent, not -0.1 percent as [Reinhart-Rogoff claim]."
Selective Exclusions. Reinhart-Rogoff use 1946-2009 as their period, with the main difference among countries being their starting year. In their data set, there are 110 years of data available for countries that have a debt/GDP over 90 percent, but they only use 96 of those years. The paper didn't disclose which years they excluded or why.

Herndon-Ash-Pollin find that they exclude Australia (1946-1950), New Zealand (1946-1949), and Canada (1946-1950). This has consequences, as these countries have high-debt and solid growth. Canada had debt-to-GDP over 90 percent during this period and 3 percent growth. New Zealand had a debt/GDP over 90 percent from 1946-1951. If you use the average growth rate across all those years it is 2.58 percent. If you only use the last year, as Reinhart-Rogoff does, it has a growth rate of -7.6 percent. That's a big difference, especially considering how they weigh the countries.

Unconventional Weighting. Reinhart-Rogoff divides country years into debt-to-GDP buckets. They then take the average real growth for each country within the buckets. So the growth rate of the 19 years that the U.K. is above 90 percent debt-to-GDP are averaged into one number. These country numbers are then averaged, equally by country, to calculate the average real GDP growth weight.

In case that didn't make sense, let's look at an example. The U.K. has 19 years (1946-1964) above 90 percent debt-to-GDP with an average 2.4 percent growth rate. New Zealand has one year in their sample above 90 percent debt-to-GDP with a growth rate of -7.6. These two numbers, 2.4 and -7.6 percent, are given equal weight in the final calculation, as they average the countries equally. Even though there are 19 times as many data points for the U.K.

Now maybe you don't want to give equal weighting to years (technical aside: Herndon-Ash-Pollin bring up serial correlation as a possibility). Perhaps you want to take episodes. But this weighting significantly reduces the average; if you weight by the number of years you find a higher growth rate above 90 percent. Reinhart-Rogoff don't discuss this methodology, either the fact that they are weighing this way or the justification for it, in their paper.
Researchers Finally Replicated Reinhart-Rogoff, and There Are Serious Problems. [Mike Konczal/Next New Deal]

Does High Public Debt Consistently Stifle Economic Growth? A Critique of Reinhart and Rogoff

(via Techdirt)

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