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# Rss talk for Bayes 250 by Steven Walker

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This is the first part of Steven's talk at Bayes 250, in connection with my discussion of the paper.

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### Rss talk for Bayes 250 by Steven Walker

1. 1. The Misspeciﬁed Bayesian Stephen Walker Stephen Walker The Misspeciﬁed Bayesian
2. 2. The talk is about a way a Bayesian can proceed without having to model big data with a big model. Can only be possible if aim is to extract something speciﬁc from the data; for example, the value of I = g(x) f0(x) dx. Choose a (misspeciﬁed) model f (x|θ) and target the θ0 for which g(x) f (x|θ0) dx = I. Prior beliefs represented by π(θ). The Bayes update may be seen as problematic. No guarantee learning is about θ0. Stephen Walker The Misspeciﬁed Bayesian
3. 3. Consider the (misspeciﬁed) Bayesian model {f (x|θ), π(θ)}. No connection between x and any θ via the probability density. Instead connect through the loss function − log f (x|θ); the target is the θ∗ which minimizes − log f (x|θ) dF0(x). Update π via a decision problem. Select ν to represent updated beliefs; i.e. minimize L(ν; x, π) = l1(ν, x) + l2(ν, π). The obvious loss functions for ν to represent revised beliefs about θ∗ are given by l1(ν, x) = − log f (x|θ) ν(dθ) and l2(ν, π) = D(ν, π). The solution to this is the Bayes update, ν(θ) ∝ f (x|θ) π(θ). Stephen Walker The Misspeciﬁed Bayesian
4. 4. The model is to do with learning about θ∗ , and the argument is essentially asymptotic. That it is all about θ∗ follows from the fact that πn(θ) accumulates at θ∗ . Hence, learning is about θ∗ . What is being learnt about does not change with the sample size. Hence, the prior is also targeting θ∗ . Need a model f (x|θ) for which targeted value θ0 and θ∗ coincide. Stephen Walker The Misspeciﬁed Bayesian
5. 5. Look at an illustration involving time series data; (xi )n i=1. Suppose interest is in learning about E0(xi xi+1), assumed to be constant for all i. Want a model f (x, y|θ) for which θ∗ , minimizing − log f (x, y|θ) f0(x, y) dx dy, and the θ0 for which x y f (x, y|θ0) dx dy = E0(x y), coincide. Stephen Walker The Misspeciﬁed Bayesian
6. 6. Such a model is provided by f (x, y|θ) = c(x, y) exp{θ x y − b(θ)}. Then b (θ0) = b (θ∗ ) = E0(x y). If we take c(x, y) = exp −1 2 (x2 + y2 ) then b (θ) = θ 1 − θ2 . Interest is in posterior distribution of r(θ) = b (θ). Stephen Walker The Misspeciﬁed Bayesian