Latent Dirichlet Allocation (Blei et al, 2003) is a powerful learning
algorithm for automatically and jointly clustering words into "topics"
and documents into mixtures of topics, and it has been successfully
applied to model change in scientific fields over time (Griffiths and
Steyver, 2004; Hall, et al. 2008). In this project, I propose to
implement a distributed variant of Latent Dirichlet Allocation using
MapReduce, and, time permitting, to investigate extensions of LDA and
possibly more efficient algorithms for distributed inference.
A topic model is, roughly, a hierarchical Bayesian model that
associates with each document a probability distribution over
"topics", which are in turn distributions over words. For instance, a
topic in a collection of newswire might include words about "sports",
such as "baseball", "home run", "player", and a document about steroid
use in baseball might include "sports", "drugs", and "politics". Note
that the labels "sports", "drugs", and "politics", are post-hoc labels
assigned by a human, and that the algorithm itself only assigns
associate words with probabilities. The task of parameter estimation
in these models is to learn both what these topics are, and which
documents employ them in what proportions.
One of the promises of unsupervised learning algorithms like Latent
Dirichlet Allocation (LDA; Blei et al, 2003) is the ability to take a
massive collections of documents and condense them down into a
collection of easily understandable topics. However, all available
open source implementations of LDA and related topics models are not
distributed, which hampers their utility. This project seeks to
correct this shortcoming.
In the literature, there have been several proposals for paralellzing
LDA. Newman, et al (2007) proposed to create an "approximate" LDA in
which each processors gets its own subset of the documents to run
Gibbs sampling over. However, Gibbs sampling is slow and stochastic by
its very nature, which is not advantageous for repeated runs. Instead,
I propose to follow Nallapati, et al. (2007) and use a variational
approximation that is fast and non-random.
David M. Blei, J McAuliffe. Supervised Topic Models. NIPS, 2007.
David M. Blei , Andrew Y. Ng , Michael I. Jordan, Latent dirichlet
allocation, The Journal of Machine Learning Research, 3, p.993-1022,
T. L. Griffiths and M. Steyvers. Finding scientiﬁc topics. Proc Natl
Acad Sci U S A, 101 Suppl 1: 5228-5235, April 2004.
David LW Hall, Daniel Jurafsky, and Christopher D. Manning. Studying
the History of Ideas Using Topic Models. EMNLP, Honolulu, 2008.
Ramesh Nallapati, William Cohen, John Lafferty, Parallelized
variational EM for Latent Dirichlet Allocation: An experimental
evaluation of speed and scalability, ICDM workshop on high performance
data mining, 2007.
Newman, D., Asuncion, A., Smyth, P., & Welling, M. Distributed
Inference for Latent Dirichlet Allocation. NIPS, 2007.
Xuerui Wang , Andrew McCallum, Topics over time: a non-Markov
continuous-time model of topical trends. KDD, 2006
Wolfe, J., Haghighi, A, and Klein, D. Fully distributed EM for very
large datasets. ICML, 2008.