Probabilistic latent semantic analysis

Probabilistic latent semantic analysis (PLSA), also known as probabilistic latent semantic indexing (PLSI, especially in information retrieval circles) is a statistical technique for the analysis of two-mode and co-occurrence data. PLSA evolved from latent semantic analysis, adding a sounder probabilistic model. PLSA has applications in information retrieval andfiltering, natural language processing, machine learning from text, and related areas. It was introduced in 1999 by Jan Puzicha and Thomas Hofmann,^[1] and it is related ^[2] to non-negative matrix factorization.

Compared to standard latent semantic analysis which stems from linear algebra and downsizes the occurrence tables (usually via asingular value decomposition), probabilistic latent semantic analysis is based on a mixture decomposition derived from a latent class model. This results in a more principled approach which has a solid foundation in statistics.

Considering observations in the form of co-occurrences (w,d) of words and documents, PLSA models the probability of each co-occurrence as a mixture of conditionally independent multinomial distributions:

P(w,d) =	∑	P(c)P(d | c)P(w | c) = P(d)	∑	P(c | d)P(w | c)
	c		c

The first formulation is the symmetric formulation, where w and d are both generated from the latent class c in similar ways (using the conditional probabilities P(d | c) and P(w | c)), whereas the second formulation is the asymmetric formulation, where, for each document d, a latent class is chosen conditionally to the document according to P(c | d), and a word is then generated from that class according to P(w | c). Although we have used words and documents in this example, the co-occurrence of any couple of discrete variables may be modelled in exactly the same way.

It is reported that the aspect model used in the probabilistic latent semantic analysis has severe overfitting problems.^[3] The number of parameters grows linearly with the number of documents. In addition, although PLSA is a generative model of the documents in the collection it is estimated on, it is not a generative model of new documents.

PLSA may be used in a discriminative setting, via Fisher kernels.^[4]

Plate notation  representing the PLSA model.   θis the document variable ( d  in the text),   z  is a topic ( c  in the text) drawn from the topic distribution for this document,   P(z | θ), and w  is a word drawn from the word distribution for this topic,   P(w | z)  . The   θ  and   w  are observable variables, the topic   z  is a   latent variable.

转自： http://en.wikipedia.org/wiki/PLSA