Chapter 14 Probabilistic Reasoning. Outline. Syntax of Bayesian networks Semantics of Bayesian networks Efficient representation of conditional distributions Exact inference by enumeration Exact inference by variable elimination Approximate inference by stochastic simulation*
Example 1： The Teeth Disease Bayesian
conditional probability table (CPT): each row contains the conditional probability of each node value for a conditioning case (a possible combination of values for the parent nodes).
A CPT for Boolean Xi with k Boolean parents has 2^k rows for the combinations of parent values. Each row requires one number p for Xi =true (the number for Xi =false is just 1-p)
If each variable has no more than k parents, The complete network requires O(n. 2^k) numbers i.e., grows linearly with n, vs. O(2^k) for the full joint distribution
For Burglary net,
Global semantics defines the full joint distribution as the product of the local conditional distributions
e.g., JohnCalls is independent of Burglary and Earthquake, given the value of Alarm.
e.g., Burglary is independent of JohnCalls and MaryCalls , given the value of Alarm and Earthquake.
Need a method such that a series of locally testable assertions of conditional independence guarantees the required global semantics.
The correct order in which to add nodes is to add the “root causes” first, then the variables they influence, and so on.
What happens if we choose the wrong order?
Q p(Q|P), p(Q| ﹁ P)
sum over hidden variables: earthquake and alarm
d = 2 when we have n Boolean variables
The complexity of variable elimination
i.e., linear in the number of variables (nodes) if the number of parents of each node is bounded by a constant
d = 2 (n Boolean variables)