CS 188: Artificial Intelligence Bayes Nets Instructor: Anca Dragan --- University of California, Berkeley [These slides were created by Dan Klein and Pieter Abbeel for CS188 Intro to AI at UC Berkeley. All CS188 materials are available at http://ai.berkeley.edu.] Probabilistic Models Models describe how (a portion of) the world works Models are always simplifications May not account for every variable May not account for all interactions between variables All models are wrong; but some are useful. George E. P. Box What do we do with probabilistic models? We (or our agents) need to reason about unknown variables, given evidence Example: explanation (diagnostic reasoning) Example: prediction (causal reasoning) Example: value of information Independence Independence
Two variables are independent if: This says that their joint distribution factors into a product two simpler distributions Another form: We write: Independence is a simplifying modeling assumption Example: Independence? T P hot 0.5 cold 0.5 T
W P T W P hot sun 0.4 hot sun 0.3 hot
rain 0.1 hot rain 0.2 cold sun 0.2 cold sun 0.3 cold
rain 0.3 cold rain 0.2 W P sun 0.6 rain 0.4 Example: Independence
N fair, independent coin flips: H 0.5 H 0.5 H 0.5 T 0.5 T 0.5 T 0.5
Conditional Independence Conditional Independence P(Toothache, Cavity, Catch) If I have a cavity, the probability that the probe catches in it doesn't depend on whether I have a toothache: P(+catch | +toothache, +cavity) = P(+catch | +cavity) The same independence holds if I dont have a cavity: P(+catch | +toothache, -cavity) = P(+catch| -cavity) Catch is conditionally independent of Toothache given Cavity: P(Catch | Toothache, Cavity) = P(Catch | Cavity) Equivalent statements: P(Toothache | Catch , Cavity) = P(Toothache | Cavity) P(Toothache, Catch | Cavity) = P(Toothache | Cavity) P(Catch | Cavity) One can be derived from the other easily Conditional Independence Unconditional (absolute) independence very rare (why?) Conditional independence is our most basic and robust form of knowledge about uncertain environments.
X is conditionally independent of Y given Z if and only if: or, equivalently, if and only if Conditional Independence Unconditional (absolute) independence very rare (why?) Conditional independence is our most basic and robust form of knowledge about uncertain environments. X is conditionally independent of Y given Z if and only if: or, equivalently, if and only if Conditional Independence What about this domain: Traffic Umbrella Raining Conditional Independence What about this domain: Fire Smoke Alarm
Conditional Independence and the Chain Rule Chain rule: Trivial decomposition: With assumption of conditional independence: Ghostbusters Chain Rule Each sensor depends only on where the ghost is That means, the two sensors are conditionally independent, given the ghost position T: Top square is red B: Bottom square is red G: Ghost is in the top Givens: P( +g ) = 0.5 P( -g ) = 0.5 P( +t | +g ) = 0.8 P( +t | -g ) = 0.4 P( +b | +g ) = 0.4 P( +b | -g ) = 0.8 P(T,B,G) = P(G) P(T|G) P(B|G)
T B G P(T,B,G) +t +b +g 0.16 +t +b -g 0.16
+t -b +g 0.24 +t -b -g 0.04 -t +b +g 0.04
-t +b -g 0.24 -t -b +g 0.06 -t -b -g 0.06
BayesNets: Big Picture Bayes Nets: Big Picture Two problems with using full joint distribution tables as our probabilistic models: Unless there are only a few variables, the joint is WAY too big to represent explicitly Hard to learn (estimate) anything empirically about more than a few variables at a time Bayes nets: a technique for describing complex joint distributions (models) using simple, local distributions (conditional probabilities) More properly called graphical models We describe how variables locally interact Local interactions chain together to give global, indirect interactions For about 10 min, well be vague about how these interactions are specified Example Bayes Net: Insurance Example Bayes Net: Car
Graphical Model Notation Nodes: variables (with domains) Can be assigned (observed) or unassigned (unobserved) Arcs: interactions Similar to CSP constraints Indicate direct influence between variables Formally: encode conditional independence (more later) For now: imagine that arrows mean direct causation (in general, they dont!) Example: Coin Flips N independent coin flips X1 X2 Xn No interactions between variables: absolute independence
Example: Traffic Variables: R: It rains T: There is traffic Model 1: independence Model 2: rain causes traffic R R T T Why is an agent using model 2 better? Example: Alarm Network Variables
B: Burglary A: Alarm goes off M: Mary calls J: John calls E: Earthquake! Example: Humans G: humans goal / humans reward parameters S: state of the physical world A: humans action G S A 23 Example: Alarm Network Variables
B: Burglary A: Alarm goes off M: Mary calls J: John calls E: Earthquake! Burglary Earthqk Alarm John calls Mary calls Example: Traffic II
Variables T: Traffic R: It rains L: Low pressure D: Roof drips B: Ballgame C: Cavity Bayes Net Semantics Bayes Net Semantics A set of nodes, one per variable X A1 An A directed, acyclic graph
A conditional distribution for each node X A collection of distributions over X, one for each combination of parents values CPT: conditional probability table A Bayes net = Topology (graph) + Local Conditional Probabilities Description of a noisy causal process Probabilities in BNs Bayes nets implicitly encode joint distributions As a product of local conditional distributions To see what probability a BN gives to a full assignment, multiply all the relevant conditionals together: Example: =P(-toothache|+cavity)P(+catch|+cavity)P(+cavity) Probabilities in BNs Why are we guaranteed that setting
results in a proper joint distribution? Chain rule (valid for all distributions): Assume conditional independences: Consequence: Example: Coin Flips X1 X2 Xn h 0.5 h 0.5 h
0.5 t 0.5 t 0.5 t 0.5 P(h)P(h)P(t)P(h) Only distributions whose variables are absolutely independent can be represented by a Bayes net with no arcs. Example: Traffic R +r
-r -t 6/16 Causality? When Bayes nets reflect the true causal patterns: Often simpler (nodes have fewer parents) Often easier to think about Often easier to elicit from experts BNs need not actually be causal Sometimes no causal net exists over the domain (especially if variables are missing) E.g. consider the variables Traffic and Drips End up with arrows that reflect correlation, not causation What do the arrows really mean? Topology may happen to encode causal structure Topology really encodes conditional independence
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