# Format of LMs

Andreas Stolcke stolcke at speech.sri.com
Sat Jan 3 19:01:24 PST 2009

Cai Hongbin-A20766 wrote:
> Hi,
> In recent days I am doing some evaluation on some SRI training tools.
> I met problems when I tried to use skipping LMs and factored LMs.
> What is the format of these models?
> As for skipping LMs, what is the meaning of the last part at the end
> of the LM file?
> \end\ ## the end of a normal LM file
> -pau- 0.5
> </s> 0.5
> <s> 0
> <unk> 0.0041594 (how to apply these coef. to some beam-search engine?)
I cannot answer the last question, but the numbers in the word list
following \end\ represent the probabilities with which a word in the
history is "skipped". So if the skip probability of a word x is p and x
occurs in a history before a word w,
the probability of w is estimated as (1-p) times the regular ngram
probability + p times the ngram probability with x removed from the history.
> As for the factored LMs, I trained a bigram, and got a result that
> there seemed to be no backing-off coef. in the unigram section.
> And what is the meaning of the coefficients right after the 2-gram probs?
> ...
> \0x0-grams:
> -1.071043 </s>
> -1.281587 <unk> (where is the backing-off coef.? )
> ...
> \0x1-grams:
> -2.178066 86AA B2BB -0.7455529(what is the meaning of these coef.?)
> -0.9450388 86AA B6BA_BAC5
> -1.72854 86AA CBF4
> -1.281777 86AA CECA_BAC5
> -6.393295 <s> </s> -0.9474632
> Anyone can show me some helpful reference? Thanks a lot.
The best documentation of FLMs can be found at
http://ssli.ee.washington.edu/people/duh/papers/flm-manual.pdf, but I
don't see an explanation of the modified backoff model file format
there. It is probably best to either read the code, or contact
bilmes at ee.washington.edu, who wrote most of the code.

Andreas