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Modelling in Classification and Statistical Learning Workshop

Suboptimality of MDL and Bayes in Classification under Misspecification

author: Peter Grünwald, National Research Institute for Mathematics and Computer Science

Description

We show that forms of Bayesian and MDL learning that are often applied to classification problems can be *statistically inconsistent*. We present a large family of classifiers and a distribution such that the best classifier within the model has generalization error (expected 0/1-prediction loss) almost 0. Nevertheless, no matter how many data are observed, both the classifier inferred by MDL and the classifier based on the Bayesian posterior will behave much worse than this best classifier in the sense that their expected 0/1-prediction loss is substantially larger. Our result can be re-interpreted as showing that under misspecification, Bayes and MDL do not always converge to the distribution in the model that is closest in KL divergence to the data generating distribution. We compare this result with earlier results on Bayesian inconsistency by Diaconis, Freedman and Barron.

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Slides
0:02 Suboptimality of Bayes and MDL in Classification
0:29 Our Result
1:17 Why is this interesting?
2:39 Menu
3:22 Classification
4:26 Classification Models
5:05 Generalization Error
6:02 Learning Algorithms
6:38 Consistent Learning Algorithms
7:57 Consistent Learning Algorithms
8:09 Main Result
12:54 Main Result
13:07 Remainder of Talk
13:40 Bayesian Learning of Classifiers
14:53 classifiers probability distrs.
16:25 Logistic transformation - intuition
17:26 Logistic transformation - intuition
18:42 Logistic transformation - intuition
18:48 classifiers probability distrs.
19:25 Logistic transformation - intuition
21:02 Logistic transformation - intuition
24:30 Main Result
24:55 Definition of .
27:21 Issues/Remainder of Talk
27:36 Scenario
29:35 Scenario – II: Definition of true D
31:16 Result:
37:20 Theorem 1
37:57 Scenario – II: Definition of true D
39:33 Theorem 1, extended
42:46 How ‘natural’ is scenario?
43:00 Scenario – II: Definition of true D
45:46 Thm 2: full Bayes result is ‘tight’
47:38 Theorem 2
49:18 Thm 2: full Bayes result is ‘tight’
49:46 Proof Sketch
51:29 Proof Sketch
53:45 Proof Sketch
54:23 Proof Sketch
55:35 Wait a minute…
58:52 Bayes predicts too well
60:11 Bayesian Consistency Results
60:47 Bayesian Consistency Results
62:58 Bayesian consistency under misspecification
63:56 Bayesian consistency under misspecification
65:21 Conclusion

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