Inference in Belief Networks

stochastic simulation

• Logic Sampling
  • randomly chose values of root-node variables according to their priors
    • continue with next layer, using CPTs

  • iterate for a number of trials

  • P(Q|E) = [freq(Q and E) / freq(Q)] in trials

• likelihood weighting
  • use given evidence values, instead of random

  • randomly choose non-evidence values

  • use CPTs to compute likelihood of this value

  • use accumulated CPTs to weight query value result

  • iterate (choosing different random values)

  • compute P(Q|E) using frequencies weighted by likelihoods