CS B553: ALGORITHMS FOR OPTIMIZATION AND LEARNINGStructure Learning

AGENDA

Learning probability distributions from example data

To what extent can Bayes net structure be learned?

Constraint methods (inferring conditional independence)

Scoring methods (learning => optimization)

BASIC QUESTION Given examples drawn from a distribution P*

with independence relations given by the Bayesian structure G*, can we recover G*?

BASIC QUESTION Given examples drawn from a distribution P*

with independence relations given by the Bayesian structure G*, can we recover G*

construct a network that encodes the same independence relations as G*?

G* G1

G2

LEARNING IN THE FACE OF NOISY DATA Ex: flip two independent coins Dataset of 20 flips: 3 HH, 6 HT, 5 TH, 6 TT

X Y

Model 1

X Y

Model 2

LEARNING IN THE FACE OF NOISY DATA Ex: flip two independent coins Dataset of 20 flips: 3 HH, 6 HT, 5 TH, 6 TT

X Y

Model 1

X Y

Model 2

ML parameters

P(X=H) = 9/20P(Y=H) = 8/20

P(X=H) = 9/20P(Y=H|X=H) = 3/9P(Y=H|X=T) = 5/11

LEARNING IN THE FACE OF NOISY DATA Ex: flip two independent coins Dataset of 20 flips: 3 HH, 6 HT, 5 TH, 6 TT

X Y

Model 1

X Y

Model 2

ML parameters

P(X=H) = 9/20P(Y=H) = 8/20

P(X=H) = 9/20P(Y=H|X=H) = 3/9P(Y=H|X=T) = 5/11 Errors are

likely to be larger!

PRINCIPLE Learning structure must trade off fit of data

vs. complexity of network Complex networks

More parameters to learn More data fragmentation = greater sensitivity to

noise

APPROACH #1: CONSTRAINT-BASED LEARNING First, identify an undirected skeleton of edges

in G*

If an edge X-Y is in G*, then no subset of evidence variables can make X and Y independent

If X-Y is not in G*, then we can find evidence variables to make X and Y independent

Then, assign directionality to preserve independences

BUILD-SKELETON ALGORITHM Given X={X1,…,Xn}, query Independent?

(X,Y,U) H = complete graph over X For all pairs Xi, Xj, test separation as follows:

Enumerate all possible separating sets U If Independent?(Xi,Xj,U) then remove Xi—Xj from

HIn practice:

• Must restrict to bounded size subsets |U|d (i.e., assume G* has bounded degree). O(n2(n-2)d) tests

• Independence can’t be tested exactly

ASSIGNING DIRECTIONALITY Note that V-structures XYZ introduce a

dependency between X and Z given Y In structures XYZ, XYZ, and XYZ, X and

Z are independent given Y In fact Y must be given for X and Z to be

independent Idea: look at separating sets for all triples X-Y-

Z in the skeleton without edge X-Z

Y

X ZTriangle

Directionality is irrelevant

ASSIGNING DIRECTIONALITY Note that V-structures XYZ introduce a

dependency between X and Z given Y In structures XYZ, XYZ, and XYZ, X and

Z are independent given Y In fact Y must be given for X and Z to be

independent Idea: look at separating sets for all triples X-Y-

Z in the skeleton without edge X-Z

Y

X ZTriangle

Y

X ZY separates X, Z

Not a v-structureDirectionality is irrelevant

ASSIGNING DIRECTIONALITY Note that V-structures XYZ introduce a

dependency between X and Z given Y In structures XYZ, XYZ, and XYZ, X and

Z are independent given Y In fact Y must be given for X and Z to be

independent Idea: look at separating sets for all triples X-Y-

Z in the skeleton without edge X-Z

Y

X ZTriangle

Y

X ZY separates X, Z

Not a v-structure

Y

X ZYU separates X, Z

A v-structureDirectionality is irrelevant

ASSIGNING DIRECTIONALITY Note that V-structures XYZ introduce a

dependency between X and Z given Y In structures XYZ, XYZ, and XYZ, X and

Z are independent given Y In fact Y must be given for X and Z to be

independent Idea: look at separating sets for all triples X-Y-

Z in the skeleton without edge X-Z

Y

X ZTriangle

Y

X ZY separates X, Z

Not a v-structure

Y

X ZYU separates X, Z

A v-structureDirectionality is irrelevant

STATISTICAL INDEPENDENCE TESTING Question: are X and Y independent? Null hypothesis H0: X and Y are independent Alternative hypothesis HA: X and Y are not

independent

STATISTICAL INDEPENDENCE TESTING Question: are X and Y independent? Null hypothesis H0: X and Y are independent Alternative hypothesis HA: X and Y are not

independent 2 test: use the statistic

withthe empirical probability of X Can compute (table lookup) the probability of

getting a value at least this extreme if H0 is true (p-value)

If p < some threshold, e.g. 1-0.95, H0 is rejected

APPROACH #2: SCORE-BASED METHODS Learning => optimization Define scoring function Score(G;D) that

evaluates quality of structure G, and optimize it Combinatorial optimization problem

Issues: Choice of scoring function: maximum likelihood

score, Bayesian score Efficient optimization techniques

MAXIMUM-LIKELIHOOD SCORES ScoreL(G;D) = likelihood of the BN with the

most likely parameter settings under structure G Let L(G,G;D) be the likelihood of data using

parameters G with structure G Let G

* = arg max L(,G;D) as described in last lecture

Then ScoreL(G;D) = L(G*,G;D)

ISSUE WITH ML SCORE

ISSUE WITH ML SCORE Independent coin example

X Y

G1

X Y

G2

ML parameters

P(X=H) = 9/20P(Y=H) = 8/20

P(X=H) = 9/20P(Y=H|X=H) = 3/9P(Y=H|X=T) = 5/11

Likelihood score

log L(G1*,G1;D)= 9 log(9/20) + 11 log(11/20) + 8 log (8/20) + 12 log (12/20)

log L(G2*,G2;D)= 9 log(9/20) + 11 log(11/20) + 3 log (3/9) + 6 log (6/9) + 5 log (5/11) + 6 log(6/11)

ISSUE WITH ML SCORE

X Y

G1

X Y

G2

Likelihood score

log L(G1*,G1;D)-log L(G2*,G2;D) = 8 log (8/20) + 12 log (12/20) – [3 log (3/9) + 6 log (6/9) + 5 log (5/11) + 6 log(6/11)]

ISSUE WITH ML SCORE

X Y

G1

X Y

G2

Likelihood score

log L(G1*,G1;D)-log L(G2*,G2;D) = 8 log (8/20) + 12 log (12/20) – 8 [3/8 log (3/9) + 5/8 log (5/11) ] – 12 [6/12 log (6/9) + 6/12 log(6/11)]

ISSUE WITH ML SCORE

X Y

G1

X Y

G2

Likelihood score

log L(G1*,G1;D)-log L(G2*,G2;D) = 8 log (8/20) + 12 log (12/20) – 8 [3/8 log (3/9) + 5/8 log (5/11) ] – 12 [6/12 log (6/9) + 6/12 log(6/11)] = 8 [log (8/20) - 3/8 log (3/9) + 5/8 log (5/11) ] + 12 [log (12/20) - 6/12 log (6/9) + 6/12 log(6/11)]

ISSUE WITH ML SCORE

X Y

G1

X Y

G2

Likelihood score

log L(G1*,G1;D)-log L(G2*,G2;D) = 8 log (8/20) + 12 log (12/20) – 8 [3/8 log (3/9) + 5/8 log (5/11) ] – 12 [6/12 log (6/9) + 6/12 log(6/11)] = 8 [log (8/20) - 3/8 log (3/9) + 5/8 log (5/11) ] + 12 [log (12/20) - 6/12 log (6/9) + 6/12 log(6/11)]=

ISSUE WITH ML SCORE

X Y

G1

X Y

G2

Likelihood score

log L(G1*,G1;D)-log L(G2*,G2;D) = 8 log (8/20) + 12 log (12/20) – 8 [3/8 log (3/9) + 5/8 log (5/11) ] – 12 [6/12 log (6/9) + 6/12 log(6/11)] = 8 [log (8/20) - 3/8 log (3/9) + 5/8 log (5/11) ] + 12 [log (12/20) - 6/12 log (6/9) + 6/12 log(6/11)]= =

ISSUE WITH ML SCORE

X Y

G1

X Y

G2

Likelihood score

log L(G1*,G1;D)-log L(G2*,G2;D) = 8 log (8/20) + 12 log (12/20) – 8 [3/8 log (3/9) + 5/8 log (5/11) ] – 12 [6/12 log (6/9) + 6/12 log(6/11)] = 8 [log (8/20) - 3/8 log (3/9) + 5/8 log (5/11) ] + 12 [log (12/20) - 6/12 log (6/9) + 6/12 log(6/11)]= =

MUTUAL INFORMATION PROPERTIES (the mutual information between X and Y)

with Q(x,y) = P(x)P(y)

0 by nonnegativity of KL divergence

Implication: ML scores do not decrease for more connected

graphs=> Overfitting to data!

POSSIBLE SOLUTIONS Fix complexity of graphs (e.g., bounded in-

degree) See HW7

Penalize complex graphs Bayesian scores

IDEA OF BAYESIAN SCORING Note that parameters are uncertain Bayesian approach: put a prior on parameter

values and marginalize them out P(D|G) =

For example, use Beta/Dirichlet priors => marginal is manageable to compute E.g., uniform hyperparameter over network Set virtual counts to 2^-|PaXi|

LARGE SAMPLE APPROXIMATION log P(D|G) =

log L(G*;D) – ½ log M Dim[G] + O(1)

With M the number of samples, Dim[G] the number of free parameters of G

Bayesian Information Criterion (BIC) score: ScoreBIC(G;D) = log L (G*;D) – ½ log M Dim[G]

LARGE SAMPLE APPROXIMATION log P(D|G) =

log L(G*;D) – ½ log M Dim[G] + O(1)

With M the number of samples, Dim[G] the number of free parameters of G

Bayesian Information Criterion (BIC) score: ScoreBIC(G;D) = log L (G*;D) – ½ log M Dim[G]

Fit data set Prefer simple models

STRUCTURE OPTIMIZATION, GIVEN A SCORE… The problem is well-defined, but

combinatorially complex! Superexponential in # of variables

Idea: search locally through the space of graphs using graph operators Add edge Delete edge Reverse edge

SEARCH STRATEGIES Greedy

Pick operator that leads to greatest score Local minima? Plateaux?

Overcoming plateaux Search with basin flooding Tabu search Perturbation methods (similar to simulated

annealing, except on data weighting) Implementation details:

Evaluate ’s between structures quickly (local decomposibility)

RECAP Bayes net structure learning: from

equivalence class of networks that encode the same conditional independences

Constraint-based methods Statistical independence tests

Score-based methods Learning => optimization