Variational Bayes via Propositionalization

We propose a unified approach to VB (variational Bayes) in symbolic-statistical modeling via propositionalization. By propositionalization we mean, broadly, expressing and computing probabilistic models such as BNs (Bayesian networks) and PCFGs (probabilistic context free grammars) in terms of propositional logic that considers propositional variables as binary random variables. Our proposal is motivated by three observations. The first one is that PPC (propostionalized probability computation), i.e. probability computation formalized in a propositional setting, has turned out to be general and efficient when variable values are sparsely interdependent. Examples include (discrete) BNs, PCFGs and more generally PRISM which is a Turing complete logic programming language with EM learning ability we have been developing, and computes probabilities using graphically represented AND/OR boolean formulas. Efficiency of PPC is classically testified by the Inside-Outside algorithm in the case of PCFGs and by recent PPC approaches in the case of BNs such as the one by Darwiche et al. that exploits $0$ probability and CSI (context specific independence). Dechter et al. also revealed that PPC is a general computation scheme for BNs by their formulation of AND/OR search spaces. Second of all, while VB has been around for sometime as a practically effective approach to Bayesian modeling, it's use is still somewhat restricted to simple models such as BNs and HMMs (hidden Markov models) though its usefulness is established through a variety of applications from model selection to prediction. On the other hand it is already proved that VB can be extended to PCFGs and is efficiently implementable using dynamic programming. Note that PCFGs are just one class of PPC and much more general PPC is realized by PRISM. Accordingly if VB is extened to PRISM's PPC, we will obtain VB for general probabilistic models, far wider than BNs and PCFGs. The last observation is that once VB becomes available in PRISM, it saves us a lot of time and energy. First we do not have to derive a new VB algorithm from scratch for each model and implement it. All we have to do is just to write a probabilistic model at predicate level. The rest of work will be carried out automatically in a unified manner by the PRISM system as it happens in the case of EM learning. Deriving and implementing a VB algorithm is a tedious error-prone process, and ensuring its correctness would be difficult beyond PCFGs without formal semantics. PRISM augmented with VB will completely eliminate such needs and make it easy to explore and test new Bayesian models by helping the user cope with data sparseness and avoid over-fitting.