CS171 - Intro to AI - Discussion Section 4

CS171 - Intro to AI - Discussion Section 4

Intro to Artificial Intelligence CS 171 Reasoning Under Uncertainty Chapter 13 and 14.1-14.2 Andrew Gelfand 3/1/2011 Today Representing uncertainty is useful in knowledge bases o Probability provides a coherent framework for uncertainty Review basic concepts in probability o Emphasis on conditional probability and conditional independence Full joint distributions are difficult to work with

o Conditional independence assumptions allow us to model real-world phenomena with much simpler models Bayesian networks are a systematic way to build structured distributions Reading: Chapter 13; beginning of Chapter 14 History of Probability in AI Early AI (1950s and 1960s) o Attempts to solve AI problems using probability met with mixed success Logical AI (1970s, 80s) o Recognized that working with full probability models is intractable o Abandoned probabilistic approaches o Focused on logic-based representations

Probabilistic AI (1990s-present) o Judea Pearl invents Bayesian networks in 1988 o Realization that working w/ approximate probability models is tractable and useful o Development of machine learning techniques to learn such models from data o Probabilistic techniques now widely used in vision, speech recognition, robotics, language modeling, game-playing, etc Uncertainty Let action At = leave for airport t minutes before flight Will At get me there on time? Problems: 1. 2. 3. 4.

partial observability (road state, other drivers' plans, etc.) noisy sensors (traffic reports) uncertainty in action outcomes (flat tire, etc.) immense complexity of modeling and predicting traffic Hence a purely logical approach either 5. 6. risks falsehood: A25 will get me there on time, or leads to conclusions that are too weak for decision making: A25 will get me there on time if there's no accident on the bridge and it doesn't rain and my tires remain intact etc etc.

(A1440 might reasonably be said to get me there on time but I'd have to stay overnight in the airport ) Handling uncertainty q Default or nonmonotonic logic: o Assume my car does not have a flat tire o Assume A25 works unless contradicted by evidence q Issues: What assumptions are reasonable? How to handle contradiction? q Rules with fudge factors: o A25 |0.3 get there on time o Sprinkler | 0.99 WetGrass o WetGrass | 0.7 Rain q Issues: Problems with combination, e.g., Sprinkler causes Rain??

q Probability o Model agent's degree of belief o Given the available evidence, o A25 will get me there on time with probability 0.04 Probability Probabilistic assertions summarize effects of o o laziness: failure to enumerate exceptions, qualifications, etc. ignorance: lack of relevant facts, initial conditions, etc. Subjective probability: q Probabilities relate propositions to agent's own state of knowledge

e.g., P(A25 | no reported accidents) = 0.06 These are not assertions about the world Probabilities of propositions change with new evidence: e.g., P(A25 | no reported accidents, 5 a.m.) = 0.15 Making decisions under uncertainty Suppose I believe the following: P(A25 gets me there on time | ) = 0.04 P(A90 gets me there on time | ) = 0.70 P(A120 gets me there on time | ) = 0.95 P(A1440 gets me there on time | ) = 0.9999 q Which action to choose? Depends on my preferences for missing flight vs. time spent waiting, etc. o Utility theory is used to represent and infer preferences

o Decision theory = probability theory + utility theory Syntax q Basic element: random variable q Similar to propositional logic: possible worlds (aka sample space) defined by assignment of values to random variables. q Boolean random variables e.g., Cavity (do I have a cavity?) q Discrete random variables e.g., Weather is one of q Domain values must be exhaustive and mutually exclusive q Elementary proposition constructed by assignment of a value to a random variable:

e.g., Weather = sunny, Cavity = false (abbreviated as cavity) q Complex propositions formed from elementary propositions and standard logical connectives e.g., Weather = sunny Cavity = false Syntax q Atomic event: A complete specification of the state of the world about which the agent is uncertain q e.g. Imagine flipping two coins o The set of all possible worlds is: S={(H,H),(H,T),(T,H),(T,T)} Meaning there are 4 distinct atomic events in this world q Atomic events are mutually exclusive and exhaustive o 2 atomic events A and B are mutually exclusive if A B = when A B

Axioms of probability q Given a set of possible worlds S o P(A) 0 for all atomic events A o P(S) = 1 o If A and B are mutually exclusive (ME), then P(A B) = P(A) + P(B) q Refer to P(A) as probability of event A q e.g. if coins are fair P({H,H}) = Probability and Logic q Probability can be viewed as a generalization of propositional logic q P(a): o a is any sentence in propositional logic

o Belief of agent in a is no longer restricted to true, false, unknown o P(a) can range from 0 to 1 P(a) = 0, and P(a) = 1 are special cases So logic can be viewed as a special case of probability Basic Probability Theory q If A and B are not ME P(A B) = P(A) + P(B) P(A B) q e.g. imagine I flip two coins o 4 possible worlds S={(H,H),(H,T),(T,H),(T,T)} and all are equally likely o Consider event E that the 1st coin is heads: E={(H,H),(H,T)} o And event F that the 2nd coin is heads: F={(H,H),(T,H)} o P(E F) = P(E) + P(F) P(E F) = + -

Conditional Probability The 2 dice problem o Suppose I roll two fair dice and 1st dice is a 4 o What is probability that sum of the two dice is 6? o 6 possible events, given 1st dice is 4 (4,1),(4,2),(4,3),(4,4),(4,5),(4,6) o Since all events (originally) had same probability, these 6 events should have equal probability o Probability that sum is 6, given 1st dice is 4 is 1/6 Conditional Probability Let A denote event that sum of dice is 6

Let B denote event that 1st dice is 4 Conditional Probability denoted as: P(A|B) o Probability of event A given event B General formula given by: o Probability of A B relative to probability of B Conditional probabilities follow same rules as unconditional or prior probabilities o e.g. P(A|B) + P(A|B) = 1 Random Variables Often interested in some function of event, rather than event itself o e.g. we care that sum of two dice is 4, not

whether the event was (1,3), (2,2) or (3,1) Random variable is a real-valued function on space of all possible worlds o e.g. Let Y = Number of heads in 2 coin flips P(Y=0) = P({T,T}) = P(Y=1) = P({H,T} {T,H}) = Prior (Unconditional) Probability q Probability distribution gives values for all possible assignments: P(Weather) Sunny 0.7

Rainy 0.1 Cloudy 0.19 Snowy 0.01 q Joint probability distribution for a set of random variables gives the probability of every atomic event on those random variables P(Weather,Cavity) Sunny

Rainy Cloudy Snowy Cavity 0.144 0.02 0.016

0.006 Cavity 0.556 0.08 0.174 0.004 q P(A,B) is shorthand for P(A B) q Joint distributions are normalized: Sa Sb P(A=a, B=b) = 1 q For a set of k binary variables, joint distribution represented as

table with 2k entries! o Desire a more compact representation Computing Probabilities Say we are given following joint distribution What is P(cavity)? Law of Total Probability (aka marginalization) P(a) = Sb P(a, b) = Sb P(a | b) P(b) Computing Probabilities What is P(cavity|toothache)? Can compute any conditional probability

given joint distribution P(c | b) = Sa Sd P(a, c, d | b) = Sa Sd P(a, c, d, b) / P(b) Computing Probabilities: The Chain Rule We can always write P(a, b, c, z) = P(a | b, c, . z) P(b, c, z) (by definition of joint probability) Repeatedly applying this idea, we can write P(a, b, c, z) = P(a | b, c, . z) P(b | c,.. z) P(c| .. z)..P(z) This factorization holds for any ordering of the variables This is the chain rule for probabilities Independence q A and B are independent iff

P(A|B) = P(A) or P(B|A) = P(B) or P(A, B) = P(A) P(B) q e.g. for n independent biased coins, O(2n) O(n) q Absolute independence is powerful but rare q e.g. Consider field of dentistry. Many variables, none of which are independent. What should we do? Conditional independence q P(Toothache, Cavity, Catch) has 23 1 = 7 independent entries q If I have a cavity, the probability that the probe catches doesn't depend on whether I have a toothache: (1) P(catch | toothache, cavity) = P(catch | cavity) q The same independence holds if I haven't got a cavity: (2) P(catch | toothache,cavity) = P(catch | cavity)

q Catch is conditionally independent of Toothache given Cavity: P(Catch | Toothache,Cavity) = P(Catch | Cavity) q Equivalent statements: P(Toothache | Catch, Cavity) = P(Toothache | Cavity) P(Toothache, Catch | Cavity) = P(Toothache | Cavity) P(Catch | Cavity) Conditional independence... q Write out full joint distribution using chain rule: P(Toothache, Catch, Cavity) = P(Toothache | Catch, Cavity) P(Catch, Cavity) = P(Toothache | Catch, Cavity) P(Catch | Cavity) P(Cavity) = P(Toothache | Cavity) P(Catch | Cavity) P(Cavity) Requires only 2 + 2 + 1 = 5 independent numbers

q In most cases, the use of conditional independence reduces the size of the representation of the joint distribution from exponential in n to linear in n. q Conditional independence is our most basic and robust form of knowledge about uncertain environments. Conditional Independence vs Independence q Conditional independence does not imply independence q Example: o A = height o B = reading ability o C = age o P(reading ability | age, height) = P(reading ability | age) o P(height | reading ability, age) = P(height | age) q Note:

o Height and reading ability are dependent (not independent) but are conditionally independent given age Bayes Rule Two urns problem o Urn 1 contains: 2 white balls & 7 black balls o Urn 2 contains: 5 white balls & 6 black balls o

Flip a fair coin and draw a ball from Urn 1 if heads; Urn 2 if tails What is probability that coin was heads, given a white ball was selected? o Want to compute P(H|W) o Have P(H) = , P(W|H) = 2/9 and P(W|H) = 5/11 Bayes' Rule q Derived from product rule: P(a b) = P(a|b) P(b) = P(b|a) P(a) P(a | b) = P(b | a) P(a) / P(b)

q or in distribution form P(Y|X) = P(X|Y) P(Y) / P(X) = P(X|Y) P(Y) q P(a | b, c) = ?? = P(b, c | a) P(a) / P(b,c) q P(a, b | c, d) = ?? = P(c, d | a, b) P(a, b) / P(c, d) Both are examples of basic pattern p(x|y) = p(y|x)p(x)/p(y) (it helps to group variables together, e.g., y = (a,b), x = (c, d)) Bayes' Rule and conditional independence q P(Cavity | toothache catch) = P(toothache catch | Cavity) P(Cavity) = P(toothache | Cavity) P(catch | Cavity) P(Cavity) q This is an example of a nave Bayes model:

P(Cause,Effect1, ,Effectn) = P(Cause) iP(Effecti|Cause) q Total number of parameters is linear in n What does all this have to do with AI? q q q Logic-based knowledge representation o Set of sentences in KB

o Agents belief in any sentence is: true, false, or unknown In real-world problems there is uncertainty o P(snow in New York on January 1) is not 0 or 1 or unknown o P(vehicle speed > 50 | sensor reading) o

P(Dow Jones will go down tomorrow | data so far) o P(pit in square 2,2 | evidence so far) o Not acknowledging this uncertainty can lead to brittle systems and inefficient use of information Uncertainty is due to: o Things we did not measure (which is always the case)

E.g., in economic forecasting o Imperfect knowledge P(symptom | disease) -> we are not 100% sure o Noisy measurements P(speed > 50 | sensor reading > 50) is not 1 Agents, Probabilities & Degrees of Belief q What we were taught in school (frequentist view) o P(a) represents the frequency that event a will happen in repeated trials -> relative

frequency interpretation q Degree of belief o P(a) represents an agents degree of belief that event a is true o This is a more general view of probability Agents probability is based on what information they have E.g., based on data or based on a theory q Examples: o a = life exists on another planet What is P(a)? We will all assign different probabilities o a = Mitt Romney will be the next US president What is P(a)? q In each case the probabilities can vary from agent to agent depending on

their models of the world and how much data they have More on Degrees of Belief q Our interpretation of P(a | e) is that it is an agents degree of belief in the proposition a, given evidence e o Note that proposition a is true or false in the real-world o P(a|e) reflects the agents uncertainty or ignorance q The degree of belief interpretation does not mean that we need new or different rules for working with probabilities o The same rules (Bayes rule, law of total probability, probabilities sum to 1) still apply our interpretation is different q If Agent 1 has inconsistent sets of probabilities (violate axioms of probability theory) then there exists a betting strategy that allows Agent 2

to always win in bets against Agent 1 o See de Finettis argument in the text Maximizing expected utility q What action should the agent take? o A rational agent should maximize expected utility, or equivalently minimize expected cost q Expected cost of actions: E[ cost(a) ] = 30 p(c) 50 [1 p(c) ] E[ cost(b) ] = -100 p(c) Break even point? 30p 50 + 50p = -100p 100p + 30p + 50p = 50 => p(c) = 50/180 ~ 0.28 If p(c) > 0.28, the optimal decision is to operate

q Original theory from economics, cognitive science (1950s) - But widely used in modern AI, e.g., in robotics, vision, game-playing q Can only make optimal decisions if know the probabilities Constructing a Propositional Probabilistic Knowledge Base q Define all variables of interest: A, B, C, Z q Define a joint probability table for P(A, B, C, Z) o We have seen earlier how this will allow us to compute the answer to any query, p(query | evidence), where query and evidence = any propositional sentence q 2 major problems: o Computation time: P(a|b) requires summing out over all other variables in the model, e.g., O(mK-1) with K variables

o Model specification Joint table has O(mK) entries where will all the numbers come from? o These 2 problems effectively halted the use of probability in AI research from the 1960s up until about 1990 Bayesian Networks A Whodunit You return home from a long day to find that your house guest has been murdered. o There are two culprits: 1) The Butler; and 2) The Cook

o There are three possible weapons: 1) A knife; 2) A gun; and 3) A candlestick Lets use probabilistic reasoning to find out whodunit? Representing the problem There are 2 uncertain quantities o Culprit = {Butler, Cook} o Weapon = {Knife, Pistol, Candlestick} What distributions should we use? o o o

o o Butler is an upstanding guy Cook has a checkered past Butler keeps a pistol from his army days Cook has access to many kitchen knives The Butler is much older than the cook Representing the problem What distributions should we use? o Butler is an upstanding guy o Cook has a checkered past P(Culprit)

o Butler keeps a pistol from his army days o Cook has access to many kitchen knives o The Butler is much older than the cook P(weapon|culprit=Butler) Pistol 0.7 Knife 0.15 Candlestick 0.15

P(weapon|culprit=Cook) Pistol 0.1 Knife 0.6 Candlestick 0.3 Butler 0.3 Cook

0.7 Solving the Crime If observe that murder weapon was a pistol, who is the most likely culprit? The Butler! Graphical Representation Culprit Weapon Each node represents a random variable Arrows indicate cause-effect relationship

Shaded nodes represent observed variables Whodunit model in words: o Culprit chooses a weapon; o You observe the weapon and infer the culprit Bayesian Networks q Represent dependence/independence via a directed graph o Nodes = random variables o Edges = direct dependence q Structure of the graph Conditional independence relations q Recall the chain rule of repeated conditioning: P(a, b, c, z) = P(a | b, c, . z) P(b | c,.. z) P(c| .. z)..P(z) p(X1, X2,....XN) = p(Xi | parents(Xi ) )

The full joint distribution The graph-structured approximation q Requires that graph is acyclic (no directed cycles) q 2 components to a Bayesian network o The graph structure (conditional independence assumptions) o The numerical probabilities (for each variable given its parents) Example of a simple Bayesian network B A p(A,B,C) = p(C|A,B)p(A)p(B) C

Probability model has simple factored form Directed edges => direct dependence Absence of an edge => conditional independence Also known as belief networks, graphical models, causal networks Other formulations, e.g., undirected graphical models Examples of 3-way Bayesian Networks A B C

Marginal Independence: p(A,B,C) = p(A) p(B) p(C) Examples of 3-way Bayesian Networks Conditionally independent effects: p(A,B,C) = p(B|A)p(C|A)p(A) B and C are conditionally independent Given A A B C

e.g., A is a disease, and we model B and C as conditionally independent symptoms given A e.g. A is culprit, B is murder weapon and C is fingerprints on door to the guests room Examples of 3-way Bayesian Networks A B Independent Causes: p(A,B,C) = p(C|A,B)p(A)p(B)

C Explaining away effect: Given C, observing A makes B less likely e.g., earthquake/burglary/alarm example A and B are (marginally) independent but become dependent once C is known Examples of 3-way Bayesian Networks A B C

Markov chain dependence: p(A,B,C) = p(C|B) p(B|A)p(A) e.g. A is culprit, B is fingerprints and C is murder weapon Example q Consider the following 5 binary variables: o B = a burglary occurs at your house o E = an earthquake occurs at your house o A = the alarm goes off o J = John calls to report the alarm o M = Mary calls to report the alarm q What is P(B | M, J) ? (for example) q Using full joint distribution to answer this question requires

o 25 = 32 probabilities q Can we use prior domain knowledge to come up with a Bayesian network that requires fewer probabilities? Constructing a Bayesian Network (1) q Order the variables in terms of causality (may be a partial order) e.g., {E, B} -> {A} -> {J, M} q P(J, M, A, E, B) = P(J, M | A, E, B) P(A| E, B) P(E, B) P(J, M | A) P(A| E, B) P(E) P(B) P(J | A) P(M | A) P(A| E, B) P(E) P(B) q These CI assumptions are reflected in the graph structure of the Bayesian

network The Resulting Bayesian Network Constructing this Bayesian Network (2) q P(J, M, A, E, B) = P(J | A) P(M | A) P(A | E, B) P(E) P(B) q There are 3 conditional probability tables (CPDs) to be determined: o o P(J | A), P(M | A), P(A | E, B) Requires 2 + 2 + 4 = 8 probabilities

q And 2 marginal probabilities P(E), P(B) -> 2 more probabilities q Where do these probabilities come from? o o o Expert knowledge From data (relative frequency estimates) Or a combination of both - see discussion in Section 20.1 and 20.2 (optional) The Bayesian network Number of Probabilities in Bayes Nets q Consider n binary variables q Unconstrained joint distribution requires O(2n) probabilities

q If we have a Bayesian network, with a maximum of k parents for any node, then we need O(n 2k) probabilities q Example o Full unconstrained joint distribution n = 30: need 109 probabilities for full joint distribution o Bayesian network n = 30, k = 4: need 480 probabilities The Bayesian Network from a different Variable Ordering The Bayesian Network from a different Variable Ordering Given a graph, can we read off conditional independencies?

A node is conditionally independent of all other nodes in the network given its Markov blanket (in gray) Inference (Reasoning) in Bayes Nets Consider answering a query in a Bayesian Network Q = set of query variables e = evidence (set of instantiated variable-value pairs) Inference = computation of conditional distribution P(Q | e) Examples P(burglary | alarm) P(earthquake | JCalls, MCalls) P(JCalls, MCalls | burglary, earthquake)

Can we use the structure of the Bayesian Network to answer queries efficiently? Answer = yes Generally speaking, complexity is inversely proportional to sparsity of graph General Strategy for inference q Want to compute P(q | e) Step 1: P(q | e) = P(q,e)/P(e) = a P(q,e) since P(e) is constant wrt Q Step 2: P(q,e) = Sa..z P(q, e, a, b, . z), by the law of total probability

Step 3: Sa..z P(q, e, a, b, . z) = Sa..z i P(variable i | parents i) (i.e. use Bayesian network factoring) Step 4: Distribute summations across product terms for efficient computation Complexity of Bayes Net inference q Assume the network is a polytree D o Only a single directed path between any 2 nodes

B q Complexity scales as O(n mK+1) n = number of variables m = arity of variables K = maximum number of parents for any node A E C o Compare to O(mn-1) for brute-force method q Network is not a polytree?

o Can cluster variables so that new graph is a tree o Complexity is then O(n mW+1), where W = # variables in largest cluster Nave Bayes Model C X1 X2 P(C | X1,Xn) = a X3 Xn

P(Xi | C) P (C) Features X are conditionally independent given the class variable C Widely used in machine learning e.g., spam email classification: Xs = counts of words in emails Probabilities P(C) and P(Xi | C) can easily be estimated from labeled data Hidden Markov Model (HMM) Y1 Y2 Y3 Yn

Observed --------------------------------------------------Hidden S3 Sn S1 S2 Two key assumptions: 1. hidden state sequence is Markov 2. observation Yt is CI of all other variables given St Widely used in speech recognition, protein sequence models Since this is a Bayesian network polytree, inference is linear in n Summary

q Bayesian networks represent joint distributions using a graph q The graph encodes a set of conditional independence assumptions q Answering queries (i.e. inference) in a Bayesian network amounts to efficient computation of appropriate conditional probabilities q Probabilistic inference is intractable in the general case o Can be done in linear time for certain classes of Bayesian networks

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