Events A and B are independent P(A) = P(A B) = P(A B) / P(B)

Transcription

1 Events A and B are independent A B U P(A) = P(A B) = P(A B) / P(B) 1

2 Alternative Characterization of Independence a) P(A B) = P(A) b) P(A B) = P(A) P(B) Recall P(A B) = P(A B) / P(B) (if P(B) 0) So P(A B) / P(B) = P(A) Multiply both sides by P(B) b) Is more obviously symmetric & avoids P(B) 0 a) Gives clearer insight into information about B being irrelevant to estimating A 2

3 Indepenences are Redundancies in the Joint Random variables A and B are Independent iff B) = P(A) B provides no information about A P(A Suppose Weather distinctions: m Fun levels: n Times of the day: k How many numbers If random variables interact? (m n k) - 1 If they are independent? (m-1) + (n-1) + (k-1) 3

4 Implications for AI Representations Joint contains just as many numbers Some are functions of others AI models should allow all and only the distinctions necessary for the task Fewer distinctions fewer numbers more efficient processing faster learning fewer inconsistencies smaller data structures 4

5 Diseases and Symptoms Are they independent? We hope not! What could be independent? Where do the symptoms come from? 5

6 Conditional Independence Model the disease influencing the symptoms But perhaps no symptom interactions given the disease Conditional independence: P(A B,C) = P(A C) A is conditionally independent of B given C 6

7 More Conditional Independence Suppose A is conditionally independent of B given C Is B necessarily conditionally independent of A given C? P(A B,C)=P(A C)? P(B A,C)=P(B C) P(A B,C) = P(A C) P(A,B,C) P(A,C) = P(B,C) P(C) P(A,B,C) P(A,C) P(B,C) = P(C) P(B A,C) = P(B C) YES, they are equivalent 7

8 More Conditional Independence A is conditionally independent of B given C C can still depend on both A and B P(C A) P(C) and P(C B) P(C) A and B are not necessarily independent: P(B A) P(B) and P(A B) P(A) But, Given C, A and B do not influence each other Knowledge of C separates A and B Suppose we do NOT know C, then A and B MAY influence each other (think about this one until it s intuitive what does influence mean here?) 8

9 Suppose that Symptoms are Conditionally Independent given the Disease We know John has a cold Congestion, a sore throat, headache, rash are more likely but not necessary P(congestion cold) >> P(congestion) likewise sore throat We discover he does in fact have a sore throat Given his cold, is he now more or less likely to also have congestion? Not much: P(congestion sorethroat, cold) ~ P(congestion cold) We may choose not to model this and other weak interactions 9

10 Bayesian Networks (belief networks, ) Graphical model Probability + Graph theory Nodes discrete random variables Directed Arcs direct influence Usually causality (class, text, ) Configuration of parents establishes contexts for a node Different distribution for each context Conditional Probability Tables (CPTs) No directed cycles (DAG) General graphical models Markov Networks (Markov random fields) Conditional random fields Dynamic Bayesian nets others 10

11 Dentist Example 3 Boolean Random Variables: C Patient has a cavity A Patient reports a toothache B Dentist s probe catches on tooth C directly influences A and B A C B A and B are conditionally independent given C CPT at each node specifies a probability distribution for each context (configuration of parents) How many numbers do we need for the full joint? How many for the Bayesian net? 11

12 How Many Numbers Joint: = 7 (why?) Bayes Net: A Distribution(s) at C P(cavity), P( cavity) Need one number Distribution(s) at A P(ache cavity), P( ache cavity) P(ache cavity), P( ache cavity) Distributions at B, like A = 5 (a savings of 2!) C B 12

13 How Many Numbers? Add binary random variables for Weather W Weekday / Weekend D How many numbers for the joint? 31 How many numbers for the BN? 7 What does the BN look like? W A C B D 13

14 How Many Numbers? Suppose A,B,C,D,W are trinary How many numbers for the joint? = 242 How many numbers for the BN? = 18 Suppose A is binary, B is trinary, C takes on 4 values D takes on 5, W takes on 6? W 5, D 4, C 3, A 4, B 8 = 24 W A C B D 14

15 Notation W, C, D, A, B are random variables C (binary presence of cavity) ranges over values C s values might be Yes or No; 1 or 0; c 1 or c 2 Probability distribution: P(C) Probability value: P(C=c 1 ) or P(c 1 ) or P(c) P(A,B,C,W,D) P(a,b,c,w,d) full joint probability distribution some particular entry in the joint 15

16 A BN represents the full Joint (it contains all of the information in the Joint*) The elements of the joint are recoverable from the BN P( x n 1, x2, x3,... x ) n i 1 P( x i parents ( X i )) (eqn 14.2) The Joint is factorable Can we represent all Joints over X 1, X 2, X n? Which cannot be represented in the BN? * Provided the conditional independences hold 16

17 How flexible is The Joint vs. A Bayesian Net Q: What Joints cannot be represented? A: Those that violate the BNs conditional independence assumptions Q: How is a conditional independence assumption denoted in a BN? A: The lack of an arc. 17

18 A Graphical Model (for us in statistical AI) A (compact) representation of a particular statistical distribution Structural or graphical Part: Commitment to random variables Observable Latent (Unobservable) Captures restrictions on how they can interact Denotes a family of distributions Parametric Part: Set of parameters (sufficient statistics) Their values individuate a distribution from the family 18