Dear Alessandro, I agree that unnecessary aggressiveness should be avoided in academic debates. Unfortunately, Potscher is not the only guy in the profession that behaves like that. Overall, my personal view if this kind of “super-unbiasedness” condition that Hansen assumes is satisfied by linear estimators only, then the BUE claim for OLS is essentially misleading. On the more general side, I believe that this story provides another piece of evidence that even very good journals may publish wrong (or, at least, useless) results by very good people. Let me remark that this conclusion would not be considered surprising at all in scientific fields different from Economics. Best, Gianluca  Da: Alessandro Casini <
> Hi, Thank you for sharing. I was aware of this. We discussed it briefly with other people in the department (Vincenzo Atella and others). Before Potscher's paper appeared online in February, Hansen and Wooldridge had exchanges on Twitter about what Potscher later showed. If you look for it on Twitter, you should be able to find the discussion. Potscher is right. What he showed was already pointed out online. So I think the way it writes is a unnecessarily aggressive. But he has always behaved in this way. It is not the first time that comes out with a takedown. Moving to the core discussion, my take is this. Hansen showed that if you drop linearity and impose further conditions on unbiasedness, then OLS is best in such class. That would mean that OLS is BUE (best unbiased estimator). Potscher showed that the restrictions on unbiasedness introduced by Hansen actually are satisfied only by linear estimators. Thus, it looks like that Hansen's result still applies to linear estimators only (implying that we go back to OLS is BLUE). However, one can interpret Potscher's results as saying that if you want unbiasedness then you need to drop non-linear estimators. Then, I see an argument to say that OLS is BUE. The discussion on Twitter was indeed on the tension between these unbiasedness restrictions and linearity/nonlinearity. Regarding the other early papers containing some related results, I do not know anything. Some of the papers Potscher referenced are written in German... Overall, I found the discussion thought-provoking. Yes, this was bad for Hansen's reputation. Potscher once again showed to be unnecessarily aggressive. I am curious to see how the discussion would go. My guess is that Ecma will publish some corrigendum from Potscher. But he has to tone down his writings. The discussion is on the process. There is a second version of Potscher's paper (after communication with Hansen) here: https://arxiv.org/pdf/2203.01425v2.pdf Let's see what happens. BTW, I presented these two papers also in my class in the PhD. Also as a lesson on how difficult is the publishing process in our profession. And that it is possible even for giants people to have mistakes in their papers (or just disagree on whether there is a mistake or not). Best, Alessandro On 5/10/2022 8:38 AM, Gianluca Cubadda wrote:
-- Alessandro Casini ( "> ) Department of Economics and Finance University of Rome Tor Vergata Via Columbia 2 00133 Rome, Italy http://alessandro-casini.com |
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