[Topic-models] Variance of corpus?

Mark Dredze mdredze at seas.upenn.edu
Wed May 20 21:48:21 EDT 2009

Hi David,

The short answer is you should look at a non-parametric version of LDA
that chooses the number of topics for you. If you want to stick with
simple LDA, then you should find a metric that matches something you
care about and optimize the number of topics. Deciding on these
metrics is non-trivial, although a number of people have chosen
metrics for doing so. You can see a new paper on evaluation metrics
for topic models that looks anew at this problem:

Hanna Wallach, Iain Murray, Ruslan Salakhutdinov, and David Mimno.
Evaluation Methods for Topic Models. ICML 2009.

However, I find the premise of your question very interesting. Your
question implies that the best number of topics is determined by some
property of variance in the corpus, ie. that there is some true number
of underlying topics that influence the number of topics used in LDA.
I think it is helpful to move away from thinking of LDA in this way
and think of it more as a general model that can be fit to the data.
Perhaps there is an underlying property of the data that determines
the best model parameters, but it is more helpful to try and optimize
these directly for some task. Ultimately, you don't care if the model
fits some true form of the data, you want a model that performs well.
Of course, defining "performs well" is the interesting challenge.


On Fri, May 15, 2009 at 6:47 PM, David André Broniatowski <david at mit.edu> wrote:
> Dear topic model list,
> Please excuse me if this is a very basic question, but I'm wondering if
> there is a principled Bayesian way to measure the variance of a corpus.
> Ultimately, I'm trying to figure out how many topics to choose in order to
> fit a topic model to a corpus.
> Thanks,
> David
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