# [Topic-models] LDA beginner's questions

Tue Nov 4 10:40:58 EST 2008

1. \alpha parameters are not known and are used only conceptually in
the generative process. They are to be estimated ?

both \alpha and \beta are to be estimated in order to maximize the likelihood of
the observed variables, these correspond to the parameters in the EM algorithm,
and \theta \z correspond to the hidden random variables in EM, for which only the
expectation of them are considered.

2. \beta parameters (word probabilities, i.e. p(w^j = 1 | z^i = 1) are
not known in advance and are to be estimated ? \beta is in fact a
matrix of conditional probabilities?

\beta is the parameter of the \z th multinomial distribution for \w.
which is also to be estimated.

3. Whenever we choose \theta from the Dirichlet(\alpha) distribution
we effectively get all but one (as N is assumed to be fixed)
parameters for the Multinomial distribution used later on ?

Sorry, I don't understand the question.

4. Why is p(z_n | \theta) given by \theta_i for the unique i such that
z_n^i = 1 ?

This is because of the expression of z, you can think of z_n as an integer i from 1 to k,
instead of a vector with only the i'th entry has the value of 1, in this way,
since z_n is generated from the multinomial distribution with one trial,
p(z_n | \theta) = \theta_(z_n) = \theta_i

5. What is the definition of a variational parameter? What are they?
(I'm completely new to variational methods)

In the E step of EM, we need to compute the posterior prob distribution of the hidden variables,
p(\theta) and p(z), they are all functions on \theta and \z, but if we consider them as a parameterized distribution,
like p(\theta | \gamma) and p(z|\phi), then the problem becomes to find the best \gamma and \phi,
this is done by minimizing the KL-divergence between like p(\theta) and p(\theta|\gamma), which is described clearly in the LDA paper.

6. What is the high level overview of the LDA algorithm?

As in other techniques using generative model, it is to make hypothesis of the model
generating the observed data, which is most often expressed in a graphical model,
then the joint probability of the observed variables are maximized in contrast to the
parameters of the model. LDA is from on pLSI, which is suitable for describing the relationship
between two kinds of discrete data, LDA uses a Dirichlet prior to describe the distribution
of the multinomial parameters as suggested by the de Finetti's theorem, which gives much better
performance, as pLSI suffers from overfitting problems, and has no generative model for new documents.

What steps do we make in order to make the LDA work correctly?

Estimate parameters and then do inference, or the other way around? I
think this is missing in the paper.

7. Is the LDA-C a 1-1 implementation of what is published in the
paper? I was trying to read the code but for the first few passes over
the code I don't see any direct mapping to most of the equations
published in the paper.

LDA-C does correspond to the LDA paper except that \alpha is constant for all the \k

Yuanlong Shao
State Key Lab of CAD&CG
Zhejiang University



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