Message Error Analysis of Loopy Belief Propagation for the Sum-Product Algorithm

Abstract

Belief propagation is known to perform extremely well in many practical statistical inference and learning problems using graphical models, even in the presence of multiple loops. The iterative use of belief propagation algorithm on loopy graphs is referred to as Loopy Belief Propagation (LBP). Various sufficient conditions for convergence of LBP have been presented; however, general necessary conditions for its convergence to a unique fixed point remain unknown. Because the approximation of beliefs to true marginal probabilities has been shown to relate to the convergence of LBP, several methods have been explored whose aim is to obtain distance bounds on beliefs when LBP fails to converge. In this paper, we derive uniform and non-uniform error bounds on messages, which are tighter than existing ones in literature, and use these bounds to derive sufficient conditions for the convergence of LBP in terms of the sum-product algorithm. We subsequently use these bounds to study the dynamic behavior of the sum-product algorithm, and analyze the relation between convergence of LBP and sparsity and walk-summability of graphical models. We finally use the bounds derived to investigate the accuracy of LBP, as well as the scheduling priority in asynchronous LBP.