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CS498-EA Reasoning in AI Lecture #5. Instructor: Eyal Amir Fall Semester 2009. Last Time. Propositional Logic Inference in different representations CNF: SAT hard; small representation sometimes DNF: SAT easy; large representation OBDDs: SAT easy; large representation sometimes

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cs498 ea reasoning in ai lecture 5

CS498-EAReasoning in AILecture #5

Instructor: Eyal Amir

Fall Semester 2009

last time
Last Time
  • Propositional Logic
    • Inference in different representations
    • CNF: SAT hard; small representation sometimes
    • DNF: SAT easy; large representation
    • OBDDs: SAT easy; large representation sometimes
    • NNF: SAT hard; fewest large representations
  • Applications:
    • Circuit and program verification; computational biology
pop quiz 5 min
Pop Quiz (5 min)
  • Prove or disprove:
    • Every CNF representation with n variables of propositional formulas takes O(2n) space to represent some propositional theories on n variables (hint: how many non-equivalent theories of n variables are there?)
  • Give me your answer; NO IMPACT on your final score in this class
today
Today
  • Probabilistic graphical models
  • Treewidth methods:
    • Variable elimination
    • Clique tree algorithm
  • Applications du jour: Sensor Networks
probability
Probability
  • A sample space Omega (O) is a set of outcomes of a random experiment
  • A probability P is a function from a sigma-field (e.g., all measurable subsets) A on O (the events) to [0,1].
  • A random variable X is a function X:OR such that for all B Borel set in R, X-1(B) is in A.
independent random variables
Independent Random Variables
  • Two variables X and Y are independent if
    • P(X = x|Y = y) = P(X = x) for all values x,y
    • That is, learning the values of Y does not change prediction of X
  • If X and Y are independent then
    • P(X,Y) = P(X|Y)P(Y) = P(X)P(Y)
  • In general, if X1,…,Xp are independent, then P(X1,…,Xp)= P(X1)...P(Xp)
    • Requires O(n) parameters
conditional independence
Conditional Independence
  • Unfortunately, most of random variables of interest are not independent of each other
  • A more suitable notion is that of conditional independence
  • Two variables X and Y are conditionally independent given Z if
    • P(X = x|Y = y,Z=z) = P(X = x|Z=z) for all values x,y,z
    • That is, learning the values of Y does not change prediction of X once we know the value of Z
    • notation: I ( X , Y | Z )
example family trees
Marge

Homer

Lisa

Maggie

Bart

Example: Family trees

Noisy stochastic process:

Example: Pedigree

  • A node represents an individual’sgenotype
  • Modeling assumptions:
    • Ancestors can effect descendants' genotype only by passing genetic materials through intermediate generations
markov assumption
Y1

Y2

X

Non-descendent

Markov Assumption

Ancestor

Parent

  • We now make this independence assumption more precise for directed acyclic graphs (DAGs)
  • Each random variable X, is independent of its non-descendents, given its parents Pa(X)
  • Formally,I (X, NonDesc(X) | Pa(X))

Non-descendent

Descendent

markov assumption example
Burglary

Earthquake

Radio

Alarm

Call

Markov Assumption Example
  • In this example:
    • I ( E, B )
    • I ( B, {E, R} )
    • I ( R, {A, B, C} | E )
    • I ( A, R | B,E )
    • I ( C, {B, E, R} | A)
i maps
X

Y

X

Y

I-Maps
  • A DAG G is an I-Map of a distribution P if all Markov assumptions implied by G are satisfied by P

(Assuming G and P both use the same set of random variables)

Examples:

factorization
X

Y

Factorization
  • Given that G is an I-Map of P, can we simplify the representation of P?
  • Example:
  • Since I(X,Y), we have that P(X|Y) = P(X)
  • Applying the chain ruleP(X,Y) = P(X|Y) P(Y) = P(X) P(Y)
  • Thus, we have a simpler representation of P(X,Y)
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