MAP estimators are not truly Bayesian estimators

This morning, I found this question from Ronald in my mailbox

“I have been reading the Bayesian Choice (2nd Edition).  On page 166 you note that, for continuous parameter spaces, the MAP estimator must be defined as the limit of a sequence of estimators  corresponding to a sequence of 0-1 loss functions with increasingly smaller nonzero epsilons.

This would appear to mean that the MAP estimator for continuous spaces is not Bayes-optimal, as is  frequently claimed.  To be Bayes-optimal, it would have to minimize the Bayes risk for a single, fixed loss function.  Instead, it must be defined using an infinite sequence of loss functions.

Does there exist a formal proof of the Bayes-optimality of the continuous-space MAP estimator-meaning one that is consistent with the usual definition assuming a fixed loss function?  I don’t see how there could be.  If a fixed loss function actually existed, then a definition requiring a limit would be unnecessary.”

which is really making a good point against MAP estimators. In fact, I have never found MAP estimators very appealing for many reasons, one being indeed that the MAP estimator cannot correctly be expressed as the solution to a minimisation problem. I also find the pointwise nature of the estimator quite a drawback: the estimator is only associated with a local property of the posterior density, not with a global property of the posterior distribution. This is in particular striking when considering the MAP estimates for two different parameterisations. The estimates often are quite different, just due to the Jacobian in the change of parameterisation. For instance, the MAP of the usual normal mean \mu under a flat prior is x, for instance x=2, but if one use a logit parameterisation instead

\mu = \log \eta/(1-\eta)

the MAP in \eta can be quite distinct from 1/(1+\exp-x), for instance leading to \mu=3 when x=2… Another bad feature is the difference between the marginal MAP and the joint MAP estimates. This is not to sat that the MAP cannot be optimal in any sense, as I suspect it could be admissible as a limit of Bayes estimates (under a sequence of loss functions). But a Bayes estimate itself?!

19 Responses to “MAP estimators are not truly Bayesian estimators”

  1. […] there is no fixed value for the parameter!) The statement that the MAP estimator is associated with the 0-1 loss function (footnote 4, p.10) is alas found in many books and papers, thus cannot truly be blamed on the […]

  2. […] surprise (or maximum profile likelihood) estimators, and loss functions. I posted a while ago my perspective on MAP estimators, followed by several comments on the Bayesian nature of those estimators, hence […]

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