KSL-94-57

## On the Approximation of Probabilistic Inference

Reference: Dagum, P. & Luby, M. On the Approximation of Probabilistic Inference. Knowledge Systems Laboratory, Medical Computer Science, September, 1994.

Abstract: Previous work proves that approximating $\Pr[X=x|E=e]$ in {\em any} sense, even for a single evidence node $E$, is {\bf NP}-hard \cite{Dagum91c}. This result holds for belief networks with {\em extreme} conditional probabilities---that is, a conditional probability $\Pr[X=x|\pi(X)]$ such that for some instantiations of the parents $\pi(X)$, $\Pr[X=x|\pi(X)]$ is arbitrarily close to zero and for other instantiations it is arbitrarily close to one. Nonetheless, all previous approximation algorithms failed to efficiently approximate many inferences even for belief networks without extreme conditional probabilities. Empirical evidence, however, suggested that the actual performance of these approximation algorithms exceeds the theoretically predicted behavior \cite{Cousins93}. Thus, two unresolved problems plagued the practicioners interested in fast approximations of inference probabilities for large belief networks. First, can we efficiently approximate $\Pr[X=x|E=e]$ for belief networks without extreme probabilities, or is this problem also {\bf NP}-hard? Second, can we prove tighter upper bounds on the run time of approximation algorithms to better predict their behavior in general belief networks. That is, can we prove bounds consistent with the empirical results observed for selected belief networks? We resolve the latter problem by proving a {\em tight} stopping rule for the number of simulations required by approximation algorithms. This result follows from the proof of the Zero-One Estimator Theorem and a similar stopping rule for zero-one valued random variables appears elsewhere \cite{Karp89}. Other previous results on a stopping rule for simulation algorithms used Bayesian statistics to derive the stopping rule \cite{Dagum91d}. Our new result does not use Bayesian statistics, and therefore, it is independent of any assumptions about the prior distribution on the mean of the estimate. The bounds are tight in the sense that an algorithm halts after a number of simulations that is within a factor of ??two of the {\em minimum} possible number of simulations. We resolve the first problem by constructing two approximation algorithms for probabilistic inference. Let $\Pr[X=x|E=e]$ denote {\em any} inference probability in a belief network without extreme conditional probabilities. The first algorithm, the {\em bounded variance algorithm}, is a variant of the known likelihood weighting algorithm \cite{Fung90a,Shachter90a}. Unlike likelihood weighting and other randomized approximation algorithms for probabilistic inference, however, we prove that the bounded variance algorithm approximates $\Pr[X=x|E=e]$ efficiently. The second algorithm uses current advances in the theory of pseudorandom generators to design a deterministic algorithm that approximates inference probabilities $\Pr[X=x|E=e]$ in worst-case time that is subexponential $2^{(\log n)^d}$, for some integer $d$. We prove that for any integer $c$ both algorithms compute {\em exactly} the first $c\log_{10} n$ significant digits of $\Pr[X=x|E=e]$. The bounded variance algorithm outputs these digits correctly in polynomial time with very high probability. The deterministic algorithm always outputs these digits correctly. Thus, in contrast to the exponential worst-case behavior of exact algorithms, we prove that if we accept a small probability of failure then we can compute inferences in polynomial worst-case time and otherwise we can {\em deterministically} compute inferences in subexponential worst-case time for belief networks without extreme conditional probabilities. We compare our results with previous work showing that if we allow extreme conditional probabilities then to even compute the first significant digit of an inference $\Pr[X=x]$ is {\bf NP}-hard \cite{Cooper90a} and to approximate in any sense the first significant digit of an inference $\Pr[X=x|E=e]$ is {\bf NP}-hard \cite{Dagum91c}.

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