First, Reinhart and Rogoff respond via email (over at Slate):
We literally just received this draft comment, and will review it in due course. On a cursory look, it seems that that Herndon Ash and Pollen also find lower growth when debt is over 90% (they find 0-30 debt/GDP, 4.2% growth; 30-60, 3.1 %; 60-90, 3.2%,; 90-120, 2.4% and over 120, 1.6%). These results are, in fact, of a similar order of magnitude to the detailed country by country results we present in table 1 of the AER paper, and to the median results in Figure 2. And they are similar to estimates in much of the large and growing literature, including our own attached August 2012 Journal of Economic Perspectives paper (joint with Vincent Reinhart) . However, these strong similarities are not what these authors choose to emphasize.
2012 JEP paper largely anticipates and addresses any concerns about aggregation (the main bone of conention here), The JEP paper not only provides individual country averages (as we already featured in Table 1 of the 2010 AER paper) but it goes further and provide episode by episode averages. Not surprisingly, the results are broadly similar to our original 2010 AER table 1 averages and to the median results that also figure prominently. It is hard to see how one can interpret these tables and individual country results as showing that public debt overhang over 90% is clearly benign.
The JEP paper with Vincent Reinhart looks at all public debt overhang episodes for advanced countries in our database, dating back to 1800. The overall average result shows that public debt overhang episodes (over 90% GDP for five years or more) are associated with 1.2% lower growth as compared to growth when debt is under 90%. (We also include in our tables the small number of shorter episodes.) Note that because the historical public debt overhang episodes last an average of over 20 years, the cumulative effects of small growth differences are potentially quite large. It is utterly misleading to speak of a 1% growth differential that lasts 10-25 years as small.
Second, Paul Krugman responds (also here):
I was going to post something sort of kind of defending Reinhart-Rogoff in the wake of the new revelations — not their results, which I never believed, nor their failure to carefully test their results for robustness, but rather their motives. But their response to the new critique is really, really bad.
What Herndon et al did was find that the R-R results on the relationship between debt and growth were partly the result of a coding error, partly the result of some very odd choices about which data to exclude and how to weight the data that remained. The effect of fixing these lapses was to raise the estimated mean growth of highly indebted countries by more than 2 percentage points.
So how do R-R respond?
First, they argue that another measure — median growth — isn’t that different from the Herndon et al results. But that is, first of all, an apples-and-oranges comparison — the fact is that when you compare the results head to head, R-R looks very off. Something went very wrong, and pointing to your other results isn’t a good defense.
Tyler Cowen's take:
2. …as Ray Lopez mentions, including in the data the postwar bouncebacks of some Anglo countries (NZ, Australia, and Canada), as recommended by the critics, is not obviously going to improve the quality of the answer. For instance the Kiwis have postwar growth rates of 7.7, 11.9, -9.9, and 10.8 percent, across the late 1940s. Are those numbers — which were combined with high postwar levels of debt — relevant to current fiscal policy issues? I say no, while admitting this may lead us to throw out other data points as well. I don’t know what is the non-cherry-pick answer here or if there even is one.
3. It is perhaps unfortunate in this age of the internet that rebuttals must be presented so quickly, but so be it. It will be interesting to hear from R&R.
4. Not too long ago I reread R&R to ascertain whether they actually present the 90% level as an emergency cliff of sorts. I concluded they did not, although there were some sentences that a reader could take out of context toward confirming such an interpretation.