7 research outputs found
IBIA: An Incremental Build-Infer-Approximate Framework for Approximate Inference of Partition Function
Exact computation of the partition function is known to be intractable,
necessitating approximate inference techniques. Existing methods for
approximate inference are slow to converge for many benchmarks. The control of
accuracy-complexity trade-off is also non-trivial in many of these methods. We
propose a novel incremental build-infer-approximate (IBIA) framework for
approximate inference that addresses these issues. In this framework, the
probabilistic graphical model is converted into a sequence of clique tree
forests (SCTF) with bounded clique sizes. We show that the SCTF can be used to
efficiently compute the partition function. We propose two new algorithms which
are used to construct the SCTF and prove the correctness of both. The first is
an algorithm for incremental construction of CTFs that is guaranteed to give a
valid CTF with bounded clique sizes and the second is an approximation
algorithm that takes a calibrated CTF as input and yields a valid and
calibrated CTF with reduced clique sizes as the output. We have evaluated our
method using several benchmark sets from recent UAI competitions and our
results show good accuracies with competitive runtimes
Exploiting Structure in Backtracking Algorithms for Propositional and Probabilistic Reasoning
Boolean propositional satisfiability (SAT) and probabilistic reasoning represent
two core problems in AI. Backtracking based algorithms have been applied in both
problems. In this thesis, I investigate structure-based techniques for solving real world
SAT and Bayesian networks, such as software testing and medical diagnosis instances.
When solving a SAT instance using backtracking search, a sequence of decisions
must be made as to which variable to branch on or instantiate next. Real world problems
are often amenable to a divide-and-conquer strategy where the original instance
is decomposed into independent sub-problems. Existing decomposition techniques
are based on pre-processing the static structure of the original problem. I propose
a dynamic decomposition method based on hypergraph separators. Integrating this
dynamic separator decomposition into the variable ordering of a modern SAT solver
leads to speedups on large real world SAT problems.
Encoding a Bayesian network into a CNF formula and then performing weighted
model counting is an effective method for exact probabilistic inference. I present two
encodings for improving this approach with noisy-OR and noisy-MAX relations. In
our experiments, our new encodings are more space efficient and can speed up the
previous best approaches over two orders of magnitude.
The ability to solve similar problems incrementally is critical for many probabilistic
reasoning problems. My aim is to exploit the similarity of these instances by
forwarding structural knowledge learned during the analysis of one instance to the
next instance in the sequence. I propose dynamic model counting and extend the dynamic
decomposition and caching technique to multiple runs on a series of problems
with similar structure. This allows us to perform Bayesian inference incrementally as
the evidence, parameter, and structure of the network change. Experimental results
show that my approach yields significant improvements over previous model counting
approaches on multiple challenging Bayesian network instances