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Belief network inference Four main approaches to determine posterior distributions in belief networks: Variable Elimination: exploit the structure of the network to eliminate (sum out) the non-observed, non-query variables one at a time.

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Belief network inference

Four main approaches to determine posterior distributions in belief networks: Variable Elimination: exploit the structure of the network to eliminate (sum out) the non-observed, non-query variables one at a time. Search-based approaches: enumerate some of the possible worlds, and estimate posterior probabilities from the worlds generated. Stochastic simulation: random cases are generated according to the probability distributions. Variational methods: find the closest tractable distribution to the (posterior) distribution we are interested in.

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  • D. Poole and A. Mackworth 2010

Artificial Intelligence, Lecture 6.4, Page 1

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Factors

A factor is a representation of a function from a tuple of random variables into a number. We will write factor f on variables X1, . . . , Xj as f (X1, . . . , Xj). We can assign some or all of the variables of a factor: f (X1 = v1, X2, . . . , Xj), where v1 ∈ dom(X1), is a factor

  • n X2, . . . , Xj.

f (X1 = v1, X2 = v2, . . . , Xj = vj) is a number that is the value of f when each Xi has value vi. The former is also written as f (X1, X2, . . . , Xj)X1 = v1, etc.

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  • D. Poole and A. Mackworth 2010

Artificial Intelligence, Lecture 6.4, Page 2

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Example factors

r(X, Y , Z): X Y Z val t t t 0.1 t t f 0.9 t f t 0.2 t f f 0.8 f t t 0.4 f t f 0.6 f f t 0.3 f f f 0.7 r(X=t, Y , Z): Y Z val t t 0.1 t f f t f f

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  • D. Poole and A. Mackworth 2010

Artificial Intelligence, Lecture 6.4, Page 3

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Example factors

r(X, Y , Z): X Y Z val t t t 0.1 t t f 0.9 t f t 0.2 t f f 0.8 f t t 0.4 f t f 0.6 f f t 0.3 f f f 0.7 r(X=t, Y , Z): Y Z val t t 0.1 t f 0.9 f t 0.2 f f 0.8 r(X=t, Y , Z=f ):

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  • D. Poole and A. Mackworth 2010

Artificial Intelligence, Lecture 6.4, Page 4

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Example factors

r(X, Y , Z): X Y Z val t t t 0.1 t t f 0.9 t f t 0.2 t f f 0.8 f t t 0.4 f t f 0.6 f f t 0.3 f f f 0.7 r(X=t, Y , Z): Y Z val t t 0.1 t f f t f f r(X=t, Y , Z=f ): Y val t f r(X=t, Y =f , Z=f ) =

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  • D. Poole and A. Mackworth 2010

Artificial Intelligence, Lecture 6.4, Page 5

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Example factors

r(X, Y , Z): X Y Z val t t t 0.1 t t f 0.9 t f t 0.2 t f f 0.8 f t t 0.4 f t f 0.6 f f t 0.3 f f f 0.7 r(X=t, Y , Z): Y Z val t t 0.1 t f f t f f r(X=t, Y , Z=f ): Y val t 0.9 f 0.8 r(X=t, Y =f , Z=f ) = 0.8

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  • D. Poole and A. Mackworth 2010

Artificial Intelligence, Lecture 6.4, Page 6

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Multiplying factors

The product of factor f1(X, Y ) and f2(Y , Z), where Y are the variables in common, is the factor (f1 × f2)(X, Y , Z) defined by: (f1 × f2)(X, Y , Z) = f1(X, Y )f2(Y , Z).

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  • D. Poole and A. Mackworth 2010

Artificial Intelligence, Lecture 6.4, Page 7

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Multiplying factors example

f1: A B val t t 0.1 t f 0.9 f t 0.2 f f 0.8 f2: B C val t t 0.3 t f 0.7 f t 0.6 f f 0.4 f1 × f2: A B C val t t t 0.03 t t f t f t t f f f t t f t f f f t f f f

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  • D. Poole and A. Mackworth 2010

Artificial Intelligence, Lecture 6.4, Page 8

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Multiplying factors example

f1: A B val t t 0.1 t f 0.9 f t 0.2 f f 0.8 f2: B C val t t 0.3 t f 0.7 f t 0.6 f f 0.4 f1 × f2: A B C val t t t 0.03 t t f 0.07 t f t 0.54 t f f 0.36 f t t 0.06 f t f 0.14 f f t 0.48 f f f 0.32

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  • D. Poole and A. Mackworth 2010

Artificial Intelligence, Lecture 6.4, Page 9

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Summing out variables

We can sum out a variable, say X1 with domain {v1, . . . , vk}, from factor f (X1, . . . , Xj), resulting in a factor on X2, . . . , Xj defined by: (

  • X1

f )(X2, . . . , Xj) = f (X1 = v1, . . . , Xj) + · · · + f (X1 = vk, . . . , Xj)

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  • D. Poole and A. Mackworth 2010

Artificial Intelligence, Lecture 6.4, Page 10

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Summing out a variable example

f3: A B C val t t t 0.03 t t f 0.07 t f t 0.54 t f f 0.36 f t t 0.06 f t f 0.14 f f t 0.48 f f f 0.32

  • B f3:

A C val t t 0.57 t f f t f f

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Artificial Intelligence, Lecture 6.4, Page 11

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Summing out a variable example

f3: A B C val t t t 0.03 t t f 0.07 t f t 0.54 t f f 0.36 f t t 0.06 f t f 0.14 f f t 0.48 f f f 0.32

  • B f3:

A C val t t 0.57 t f 0.43 f t 0.54 f f 0.46

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Artificial Intelligence, Lecture 6.4, Page 12

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Exercise

Given factors: s: A val t 0.75 f 0.25 t: A B val t t 0.6 t f 0.4 f t 0.2 f f 0.8

  • :

A val t 0.3 f 0.1 What is? (a) s × t (b)

A s × t

(c)

B s × t

(d) s × t × o (e)

A s × t × o

(f)

b s × t × o

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  • D. Poole and A. Mackworth 2010

Artificial Intelligence, Lecture 6.4, Page 13

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Evidence

If we want to compute the posterior probability of Z given evidence Y1 = v1 ∧ . . . ∧ Yj = vj: P(Z|Y1 = v1, . . . , Yj = vj) =

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  • D. Poole and A. Mackworth 2010

Artificial Intelligence, Lecture 6.4, Page 14

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Evidence

If we want to compute the posterior probability of Z given evidence Y1 = v1 ∧ . . . ∧ Yj = vj: P(Z|Y1 = v1, . . . , Yj = vj) = P(Z, Y1 = v1, . . . , Yj = vj) P(Y1 = v1, . . . , Yj = vj) =

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  • D. Poole and A. Mackworth 2010

Artificial Intelligence, Lecture 6.4, Page 15

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Evidence

If we want to compute the posterior probability of Z given evidence Y1 = v1 ∧ . . . ∧ Yj = vj: P(Z|Y1 = v1, . . . , Yj = vj) = P(Z, Y1 = v1, . . . , Yj = vj) P(Y1 = v1, . . . , Yj = vj) = P(Z, Y1 = v1, . . . , Yj = vj)

  • Z P(Z, Y1 = v1, . . . , Yj = vj).

So the computation reduces to the probability of P(Z, Y1 = v1, . . . , Yj = vj). We normalize at the end.

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Artificial Intelligence, Lecture 6.4, Page 16

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Probability of a conjunction

Suppose the variables of the belief network are X1, . . . , Xn. To compute P(Z, Y1 = v1, . . . , Yj = vj), we sum out the other variables, Z1, . . . , Zk = {X1, . . . , Xn} − {Z} − {Y1, . . . , Yj}. We order the Zi into an elimination ordering. P(Z, Y1 = v1, . . . , Yj = vj) =

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  • D. Poole and A. Mackworth 2010

Artificial Intelligence, Lecture 6.4, Page 17

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Probability of a conjunction

Suppose the variables of the belief network are X1, . . . , Xn. To compute P(Z, Y1 = v1, . . . , Yj = vj), we sum out the other variables, Z1, . . . , Zk = {X1, . . . , Xn} − {Z} − {Y1, . . . , Yj}. We order the Zi into an elimination ordering. P(Z, Y1 = v1, . . . , Yj = vj) =

  • Zk

· · ·

  • Z1

P(X1, . . . , Xn)Y1 = v1,...,Yj = vj. =

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  • D. Poole and A. Mackworth 2010

Artificial Intelligence, Lecture 6.4, Page 18

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Probability of a conjunction

Suppose the variables of the belief network are X1, . . . , Xn. To compute P(Z, Y1 = v1, . . . , Yj = vj), we sum out the other variables, Z1, . . . , Zk = {X1, . . . , Xn} − {Z} − {Y1, . . . , Yj}. We order the Zi into an elimination ordering. P(Z, Y1 = v1, . . . , Yj = vj) =

  • Zk

· · ·

  • Z1

P(X1, . . . , Xn)Y1 = v1,...,Yj = vj. =

  • Zk

· · ·

  • Z1

n

  • i=1

P(Xi|parents(Xi))Y1 = v1,...,Yj = vj.

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  • D. Poole and A. Mackworth 2010

Artificial Intelligence, Lecture 6.4, Page 19

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Computing sums of products

Computation in belief networks reduces to computing the sums of products. How can we compute ab + ac efficiently?

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  • D. Poole and A. Mackworth 2010

Artificial Intelligence, Lecture 6.4, Page 20

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Computing sums of products

Computation in belief networks reduces to computing the sums of products. How can we compute ab + ac efficiently? Distribute out the a giving a(b + c)

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  • D. Poole and A. Mackworth 2010

Artificial Intelligence, Lecture 6.4, Page 21

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Computing sums of products

Computation in belief networks reduces to computing the sums of products. How can we compute ab + ac efficiently? Distribute out the a giving a(b + c) How can we compute

Z1

n

i=1 P(Xi|parents(Xi))

efficiently?

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  • D. Poole and A. Mackworth 2010

Artificial Intelligence, Lecture 6.4, Page 22

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Computing sums of products

Computation in belief networks reduces to computing the sums of products. How can we compute ab + ac efficiently? Distribute out the a giving a(b + c) How can we compute

Z1

n

i=1 P(Xi|parents(Xi))

efficiently? Distribute out those factors that don’t involve Z1.

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  • D. Poole and A. Mackworth 2010

Artificial Intelligence, Lecture 6.4, Page 23

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Variable elimination algorithm

To compute P(Z|Y1 = v1 ∧ . . . ∧ Yj = vj): Construct a factor for each conditional probability. Set the observed variables to their observed values. Sum out each of the other variables (the {Z1, . . . , Zk}) according to some elimination ordering. Multiply the remaining factors. Normalize by dividing the resulting factor f (Z) by

Z f (Z).

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Artificial Intelligence, Lecture 6.4, Page 24

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Summing out a variable

To sum out a variable Zj from a product f1, . . . , fk of factors: Partition the factors into

◮ those that don’t contain Zj, say f1, . . . , fi, ◮ those that contain Zj, say fi+1, . . . , fk

We know:

  • Zj

f1× · · · ×fk = f1× · · · ×fi×  

Zj

fi+1× · · · ×fk   . Explicitly construct a representation of the rightmost

  • factor. Replace the factors fi+1, . . . , fk by the new factor.

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Artificial Intelligence, Lecture 6.4, Page 25

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Variable elimination example

A B C D E F G H I P(A) P(B|A)

  • elim A

− → f1(B) P(C) P(D|BC) P(E|C)    elim C − → f2(BDE) P(F|D) P(G|FE) P(H|G)

  • bs H

− → f3(G) P(I|G) elim I − → f4(G) P(D, h) = ...(

A P(A)P(B|A))( I P(I|G))

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Artificial Intelligence, Lecture 6.4, Page 26

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Variable Elimination example

A B C D E F G H

Query: P(G|f ); elimination ordering: A, H, E, D, B, C P(G|f ) ∝

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Variable Elimination example

A B C D E F G H

Query: P(G|f ); elimination ordering: A, H, E, D, B, C P(G|f ) ∝

  • C
  • B
  • D
  • E
  • H
  • A

P(A)P(B|A)P(C|B) P(D|C)P(E|D)P(f |E)P(G|C)P(H|E)

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Artificial Intelligence, Lecture 6.4, Page 28

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Variable Elimination example

A B C D E F G H

Query: P(G|f ); elimination ordering: A, H, E, D, B, C P(G|f ) ∝

  • C
  • B
  • D
  • E
  • H
  • A

P(A)P(B|A)P(C|B) P(D|C)P(E|D)P(f |E)P(G|C)P(H|E) =

  • C
  • B
  • A

P(A)P(B|A)

  • P(C|B)
  • P(G|C)
  • D

P(D|C)

  • E

P(E|D)P(f |E)

  • H

P(H|E)

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Artificial Intelligence, Lecture 6.4, Page 29