[SRILM User List] Class-based probability using -expand-classes

Andreas Stolcke stolcke at icsi.berkeley.edu
Wed May 16 14:33:23 PDT 2018

I'm not sure what "an approximate inverse to the original language 
model" means.

But the purpose of ngram -expand-classes is to approximate the class LM 
probabilities (the equation you give) using only word ngram 
probabilities.  It does so by inserting all expanded word ngrams into 
the LM and giving them probabilities according to

P(w_n | w_n-1 ... w_1) = P(w_n w_n-1 ... w_1) / P(w_n-1 ... w_1)

where the joint probabilities on the right hand side are computed by the 
class-based LM.


On 5/16/2018 1:35 PM, Jonathan Mendoza wrote:
> SRILM community,
> If I build a class based LM via
> replace-words-with-classes -> ngram
> then re-build the LM using -expand-classes,
> will the rebuilt LM follow the class based probabilities,
> P(w_n | w_n-1 ... w_1) ?= P(w_n | c_n) * P(c_n | c_n-1 ... c_1).
> Or is the mapping an approximate inverse to the original language model?
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