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Tutorial #9 by Ma’ayan Fishelson

Bucket Elimination. Tutorial #9 by Ma’ayan Fishelson. Bucket Elimination Algorithm. An algorithm for performing inference in a Bayesian network. Similar algorithms can be used in other domains, such as constraint satisfaction. Bayesian Network.

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Tutorial #9 by Ma’ayan Fishelson

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  1. Bucket Elimination Tutorial #9 by Ma’ayan Fishelson

  2. Bucket Elimination Algorithm • An algorithm for performing inference in a Bayesian network. • Similar algorithms can be used in other domains, such as constraint satisfaction.

  3. Bayesian Network • X = {X1,…,Xn} is a set of random variables. • A BN is a pair (G,P): • G is a directed acyclic graph over nodes that represent the random variables X. • P = {Pi|1 ≤i ≤n}. Pi, defined on , is the conditional probability table associated with node Xi. Pi = P(Xi | pa(Xi)) • The BN represents a probability distribution over X.

  4. Bayesian Network Moralized Graph A A P(A) B C B C P(B|A) P(C|A) E E P(E|B,C) D D P(D|A,B) F F P(F|E) Bayesian Network - Example P(A,B,C,D,E,F) = P(A)P(B|A)P(C|A)P(D|A,B)P(E|B,C)P(F|E)

  5. Some Probabilistic Inference Tasks… • Belief Assessment: find P(Xi = xi | e). • Most Probable Explanation (MPE): find

  6. Solution – Bucket Elimination Algorithms Given an ordering of the variables X1,…Xn: • Distribute P1,…,Pn into buckets B1,…,Bn. is the highest in order in Ai. • Process the buckets in reverse order: BnB1. (Whenprocessing bucket Bi, multiply all the probability tables in Bi and eliminate the bucket’s variable Xi by summing over all its possible values.) • Place the resulting function in the bucket of the highest variable (in the order) that is in its scope.

  7. Example – Belief Assessment • Compute: • P(A=a) = Σ{A=a,B,C,D,E,F}P(A,B,C,D,E,F) = • Σ{A=a,B,C,D,E,F}P(A)P(B|A)P(C|A)P(D|A,B)P(E|B,C)P(F|E) • Suppose an order A,C,B,E,D,F. and evidence that F=1. • The distribution into buckets is as follows: P(A) P(C|A) P(B|A) P(E|B,C) P(D|A,B) P(F|E) B2(C) B3(B) B4(E) B5(D) B6(F) B1(A)

  8. P(A) P(C|A) P(B|A) P(E|B,C) P(D|A,B) P(F|E) B2(C) B3(B) B4(E) B5(D) B6(F) B1(A) P(A) P(C|A) P(B|A) P(E|B,C) λF(E) P(D|A,B) B2(C) B3(B) B4(E) B5(D) B6(F) B1(A) Example – Computing P(A=a) • To process B6: Assign F=1, get λF(E) = P(F=1|E) • Place λF(E) in bucket B4(E).

  9. P(A) P(C|A) P(B|A) P(E|B,C) λF(E) P(D|A,B) B2(C) B3(B) B4(E) B5(D) B6(F) B1(A) P(B|A) λD(A,B) P(A) P(C|A) P(E|B,C) λF(E) B2(C) B3(B) B4(E) B5(D) B6(F) B1(A) Computing P(A=a) (cont. 1) • To process B5: λD(A,B) = ΣDP(D|A,B) • Place λD(A,B) in bucket B3(B).

  10. P(A) P(C|A) P(B|A) λD(A,B) P(E|B,C) λF(E) B2(C) B3(B) B4(E) B5(D) B6(F) B1(A) P(B|A) λD(A,B) λE(B,C) P(A) P(C|A) B2(C) B4(E) B5(D) B6(F) B1(A) B3(B) Computing P(A=a) (cont. 2) • To process B4: λE(B,C) = ΣEP(E|B,C)λF(E) • Place λE(B,C) in bucket B3(B).

  11. P(B|A) λD(A,B) λE(B,C) P(A) P(C|A) B2(C) B4(E) B5(D) B6(F) B1(A) B3(B) P(C|A) λB(A,C) P(A) B2(C) B3(B) B4(E) B5(D) B6(F) B1(A) Computing P(A=a) (cont. 3) • To process B3: λB(A,C) = ΣBP(B|A)λD(A,B) λE(B,C) • Place λB(A,C) in bucket B2(C).

  12. P(A) P(C|A) λB(A,C) B2(C) B4(E) B5(D) B6(F) B3(B) B1(A) P(A) λC(A) B2(C) B3(B) B4(E) B5(D) B6(F) B1(A) Computing P(A=a) (cont. 4) • To process B2: λC(A) = ΣCP(C|A)λB(A,C) • Place λC(A) in bucket B1(A).

  13. P(A) λC(A) B2(C) B3(B) B4(E) B5(D) B6(F) B1(A) Computing P(A=a) (cont. 5) • To compute the belief P(A=a), we multiply P(A)λC(A). • We obtain a function of A.

  14. B E A C D D C B C E B A A D E Order B,C,D,E,A. w*(d1) =4 Order E,D,C,B,A. w*(d2) =2 Moral graph Bucket Elimination Complexity • The time and space complexity is bounded by the size of the probability tables that are created by the algorithm. • w*(d) is the induced width (max clique size) of the moral graph along ordering d.

  15. Constraint Satisfaction Problem (CSP) • Input: • A set of variables: X = {X1,…,Xn}. • A set of constraints: R = {R1,…,Rm}. • For each constraint Ri: • Scope of Ri: • Ri is a relation over the variables in Ai. • Output: • An assignment to X that is consistent with R.

  16. Constraint Graph • Nodes: {X1,…Xn}. • Edges: For simplicity, assume that constraints are defined on maximal cliques of the graph.

  17. A A1/A1 O A2/A2 1 2 A A1/A2 A A2/A2 3 4 O A1/A2 5 Example of CSP: Find Possible Genotypes A pedigree typed at the ABO and AK1 loci: • Possible haplotypes: h1=A,A1; h2 = A,A2; h3=O,A1; h4 = O,A2; • Possible genotypes (ordered): gij = hi/hj(father/mother) Ind #1: g11, g13, g31 Ind #2: g44 Ind #3: g12, g21, g14, g41, g23, g32 Ind #4: g22, g24, g42 Ind #5: g34, g43 Constraints: R13 = {<g11,g{12,14}>, <g{13,31},g{12,14,32}>}, similarly for R23, R35, and R45.

  18. Bucket Elimination for CSP • Given an order X1,…, Xn: • Distribute constraints into buckets, according to the highest variable in the constraint’s scope. • Process the buckets one by one starting from Xn: • Join the constraints (with equal value for the eliminated variable), in the eliminated bucket and remember the value of the eliminated variable. • Put the new constraint in the bucket of the highest variable in its scope.

  19. A B C D E Constraint graph Example of CSP: Find Possible Genotypes (cont. 1) Constraints: • RAC = {<g11,g{12,14}>, <g{13,31},g{12,14,32}>}; • RBC = {<g44, g14>}; • RCE = {<g{14,41,23,32} g{34,43}}; • RDE = {<g{24,42} g34>}

  20. Find Possible Genotypes (cont. 2) • Suppose an order: A, B, C, D, E. • The distribution into buckets is as follows:

  21. Find Possible Genotypes (cont. 3) • When processing BE: • compute RCD = {<g{23,32,14,41}, g{24,42}>}. • put RCD into bucket BD. • remember the value g34 for E.

  22. Find Possible Genotypes (cont. 4) • When processing BD: • compute RC = {<g{23,32,14,41}}. • put RC into bucket BC. • remember the values g24,g42 for D.

  23. Find Possible Genotypes (cont. 5) • When processing BC: • compute RAB = {<g{11,13, 31}, g44>}. • put RC into bucket BB. • remember the value g14 for C.

  24. Find Possible Genotypes (cont. 6) • When processing BB: • compute RA = {<g{11,13, 31}>}. • remember the value g44 for B.

  25. A A1/A1 O A2/A2 1 2 A A1/A2 A A2/A2 3 4 O A1/A2 5 A|A or A|O or O|A A1|A1 O|O A2|A2 1 2 A|O A1|A2 A|O or O|A A2/A2 3 4 O|O A1|A2 5 Find Possible Genotypes (cont. 7) • By backtracking we get the • possible values: • Ind #1 (A): g11, g13, g31 • Ind #2 (B): g44 • Ind #3 (C): g14 • Ind #4 (D): g24, g42 • Ind #5 (E): g34

  26. Bucket Elimination Algorithm

  27. Bucket Elimination Algorithm

  28. Bucket Elimination Algorithm • More variations of the algorithm • http://www.ics.uci.edu/~csp/R48a.pdf

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