“How Better Information Can Garble Experts’ Advice,” M. Elliott, B. Golub & A. Kirilenko (2012)

Ben Golub from Stanford GSB is on the market this year following a postdoc. This paper, which I hear is currently under submission, is a simple and straightforward theoretical point, but it does have some worrying implications for public policy. Consider a set of experts which society queries about the chance of some probabalistic event; the authors mention the severity of a flu, or the risk of a financial crisis, as examples. These experts all have different private information. Given their private information, and the (unmodeled) payoff they receive from a prediction, they weigh the risk of type I and type II errors.

Now imagine that information improves for each expert (restricting to two experts as in the paper). With the new information, any possible set of type I and type II errors is still possible, and there is now the possibility of making predictions with strictly fewer type I and type II errors. This means that the “error frontier” expands outward for each expert. To be precise, if each agent gets a signal in [0,1] whose cdf is G(i) for expert i if the event will actually occur. A new signal that generates a second cdf G2(i) which first order stochastically dominates G(i) is an information improvements. Imagine both experts receive information improvements. Is this socially useful? It turns out that is it not necessarily a good thing.

How? Imagine that expert 1 is optimizing by making x1 type I errors and y1 type II errors given his signal, and expert 2 is optimizing by making x2 type I errors and y2 type II errors. Initially expert 1 is making very few type I errors, and expert 2 is making very few type II errors. Information improves for both, pushing out the “error frontier”. At the new optimum for expert 1, he makes more type I errors, but many fewer type II errors. Likewise, at the new optimum, expert 2 makes more type II errors and fewer type I errors. Indeed, it can be the case that expert I after the information improvement is making more type I and type II errors than expert 2 did in her original prediction, and that expert II is now making more type I and type II errors than expert 1 did in his original prediction. That is, the new set of predictions are a Blackwell garbling of the original set of predictions, and hence less useful to society no matter what decision rule society uses when applying the information to some problem. Note that this result does not depend on experts trying to out-guess each other or anything similar.

Is such a perverse outcome unusual? Not necessarily. Let both experts be “envious” before new information arrives, meaning the both experts prefer the other’s bundle of type I and type II errors to any such bundle the expert can choose himself. Let the agents payoffs not depend on the prediction of the other agents. Finally, Let the new information be a “technology transfer”, meaning a sharing of some knowledge already known to one or both agents. That is, after the new information arrives, the error frontier of both agents lies within the convex hull of their original combined error frontiers. With envious agents, there is always a technology transfer that makes society worse off. All of the above holds even when experts are not required to make discrete {0,1} predictions.

This is all to say that, as the authors note, “better diagnostic technology need not lead to better diagnoses”. But let’s not go too far: there is no principal-agent game here. You may wonder if society can design payment rules to experts to avoid such perversity. We have a literature, now large, on expert testing, where you want to avoid paying “fake experts” for information. Though you can’t generally tell experts and charlatans apart, Shmaya and Echenique have a paper showing that there do exist mechanisms to ensure that, at least, I am not harmed “too much” by the charlatans’ advice. It is not clear whether a mechanism exists for paying experts which ensures that information improvements are strictly better for society. By Blackwell’s theorem, more information is strictly better for the principal, so incentivizing the experts to express their entire type I-type II error frontier (which is equivalent to expressing their prior and their signal) would work. How to do that is a job for another paper.

July 2012 working paper (unavailable on Repec IDEAS).


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