Space: Apache Mahout (https://cwiki.apache.org/confluence/display/MAHOUT)
Page: Latent Dirichlet Allocation
(https://cwiki.apache.org/confluence/display/MAHOUT/Latent+Dirichlet+Allocation)
Edited by Federico Castanedo:
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h1. Overview
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. It has been successfully applied to model change in
scientific fields over time (Griffiths and Steyvers, 2004; Hall, et al. 2008).
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 the topics are, and which documents employ them in
what proportions.
Another way to view a topic model is as a generalization of a mixture model
like [Dirichlet Process Clustering]. Starting from a normal mixture model, in
which we have a single global mixture of several distributions, we instead say
that _each_ document has its own mixture distribution over the globally shared
mixture components. Operationally in Dirichlet Process Clustering, each
document has its own latent variable drawn from a global mixture that specifies
which model it belongs to, while in LDA each word in each document has its own
parameter drawn from a document-wide mixture.
The idea is that we use a probabilistic mixture of a number of models that we
use to explain some observed data. Each observed data point is assumed to have
come from one of the models in the mixture, but we don't know which. The way
we deal with that is to use a so-called latent parameter which specifies which
model each data point came from.
h1. Invocation and Usage
Mahout's implementation of LDA operates on a collection of SparseVectors of
word counts. These word counts should be non-negative integers, though things
will-- probably --work fine if you use non-negative reals. (Note that the
probabilistic model doesn't make sense if you do!) To create these vectors,
it's recommended that you follow the instructions in [Creating Vectors From
Text], making sure to use TF and not TFIDF as the scorer.
Invocation takes the form:
{noformat}
bin/mahout lda \
-i <input vectors directory> \
-o <output working directory> \
-k <numTopics> \
-v <number of words> \
-a <optional topic smoothing. Default: 50/numTopics> \
-x <optional number of iterations. Default is -1 (until convergence)> \
{noformat}
Topic smoothing should generally be about 50/K, where K is the number of
topics. The number of words in the vocabulary can be an upper bound, though it
shouldn't be too high (for memory concerns).
Choosing the number of topics is more art than science, and it's recommended
that you try several values.
After running LDA you can obtain an output of the computed topics using the
LDAPrintTopics utility:
{noformat}
bin/mahout ldatopics \
-i <input vectors directory> \
-d <input dictionary file> \
-o <optional output working directory. Default is to console> \
-dt <optional dictionary type (text|sequenceFile). Default is text>
{noformat}
h1. Example
An example is located in mahout/examples/bin/build-reuters.sh. The script
automatically downloads the Reuters-21578 corpus, builds a Lucene index and
converts the Lucene index to vectors. By uncommenting the last two lines you
can then cause it to run LDA on the vectors and finally print the resultant
topics to the console.
To adapt the example yourself, you should note that Lucene has specialized
support for Reuters, and that building your own index will require some
adaptation. The rest should hopefully not differ too much.
h1. Parameter Estimation
We use mean field variational inference to estimate the models. Variational
inference can be thought of as a generalization of [EM|Expectation
Maximization] for hierarchical Bayesian models. The E-Step takes the form of,
for each document, inferring the posterior probability of each topic for each
word in each document. We then take the sufficient statistics and emit them in
the form of (log) pseudo-counts for each word in each topic. The M-Step is
simply to sum these together and (log) normalize them so that we have a
distribution over the entire vocabulary of the corpus for each topic.
In implementation, the E-Step is implemented in the Map, and the M-Step is
executed in the reduce step, with the final normalization happening as a
post-processing step.
h1. References
[David M. Blei, Andrew Y. Ng, Michael I. Jordan, John Lafferty. 2003. Latent
Dirichlet Allocation. JMLR.|
http://www.cs.princeton.edu/~blei/papers/BleiNgJordan2003.pdf]
[Thomas L. Griffiths and Mark Steyvers. 2004. Finding scientific topics. PNAS.
|http://psiexp.ss.uci.edu/research/papers/sciencetopics.pdf]
[David Hall, Dan Jurafsky, and Christopher D. Manning. 2008. Studying the
History of Ideas Using Topic Models |
http://www.aclweb.org/anthology-new/D/D08/D08-1038.pdf]
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