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Graphical Models BRML Chapter 4 1

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the zoo of graphical models Markov networks Belief networks Chain graphs (Belief and Markov ) Factor graphs =>they are graphical models 2

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two steps of process Modeling identify the relevant variables make structural assumptions Inference answer the interest questions with inference algorithm 3

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Markov Networks Factorization the joint probability is described with the product of several potential functions Potential a non-negative function of the set of variables This is not probability itself. (Fig4.1) 4

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What is maximal cliques? cliques:a fully connected subset of nodes maximal cliques:cliques which cant’t be extended by including one more adjecent node We define potential functions on each maximal cliques >Hammersley-Clifford Theorem 5

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Properties of Markov Networks Definition4.4 marginalizing,conditioning Definition4.5 separation (Fig4.2) 6

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Lattice model Ising model ->magnet, image analysis 7

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Chain Graphical Models Chain Graphs contain both directed and undirected links.(Fig4.6,4.7) 1)Remove directed edges 2)Consider the distributions over maximal clique 3)Moralize and normalize 8

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Factor Graphs(Fig4.8) Factor graphs are used for inference steps. Square nodes represent factors. ex)p(A,B,C)=P(A)P(B)P(C|A,B) C BA 9

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Example of factor graphs f(A,B,C)=P(A)P(B)P(C|A,B) A B C f Variable node Factor node 10

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Example of factor graph (2) AB C f1=P(A),f2=P(B),f3=P(C|A,B) => P(A,B,C)=f1*f2*f3 f1 f3 f2 11

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Expressiveness of Graphical Models Directed distributions can be represented as undirected ones. But this transformation can lose some information(conditional independency) the converse questions are same. So,we need I-map D-map perfect-map(see Definition4.11) 12

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references BRML chapter 4 PRML chapter 8 13

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Graduate School of Information Sciences, Tohoku University

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