Hi Adam and Nicola, Ah that explains it.. great thanks for the quick responses! (and was using SRI LM by the way) Kevin
On Fri, Feb 5, 2010 at 8:01 AM, Nicola Bertoldi <[email protected]> wrote: > Dear Kevin, > > the recombination takes into account the history that the LM ACTUALLY uses > for the computation of the prob. > > It can happen that the LM does not know any 4-grams with history ", beer a" > or "<s> beer a", > hence the LM makes always a backoff on the common history "beer a". > > Could you check please if your LM has shown any 4-grams with history ", > beer a" or "<s> beer a"? > Which type of LM are you using (SRI or IRST)? > > Nicola > > > On 2/5/10 1:47 PM, "Kevin Gimpel" <[email protected]> wrote: > > Hey all, > > I'm trying to construct a phrase lattice as output from Moses. I have been > playing around with "-output-search-graph" and "-verbose 3" and have become > confused about recombination and how it preserves language model states. > > For example, if I translate "hier ist ein bier" from German to English and > use a 4-gram language model, I see the following lines as part of the output > when using -output-search-graph: > > ... > 0 hyp=17 stack=1 back=0 score=-5.57705 transition=-5.57705 forward=35 > fscore=-10.8062 covered=3-3 out=beer , pC=0.131725, c=-2.6988 > 0 hyp=18 stack=1 back=0 score=-8.39914 transition=-8.39914 forward=50 > fscore=-11.1884 covered=3-3 out=, beer , pC=-1.81449, c=-5.14484 > ... > 0 hyp=47 stack=2 back=17 score=-11.4177 transition=-5.84061 forward=173 > fscore=-12.6408 covered=2-2 out=a , pC=-0.318772, c=-1.8764 > 0 hyp=62 stack=2 back=18 score=-13.6186 transition=-5.2195 recombined=47 > forward=173 fscore=-12.6408 covered=2-2 out=a , pC=-0.318772, c=-1.8764 > > I am surprised that recombination occurs in the last line shown, because > hypothesis 62 ends in ", beer a" while hypothesis 47 ends in "<s> beer a" -- > causing future hypotheses that come from 47 or 62 to have different 4-gram > language model probabilities. I had been thinking that recombination was a > risk-free pruning method of the search space as described in the Moses > background page / original Pharaoh paper ( > http://www.statmt.org/moses/?n=Moses.Background), but maybe my assumption > is obsolete. > > I can see a couple possibilities here: > 1. Moses checks all necessary LM probabilities for the given trailing > trigrams in each hypothesis and determines that the recombination can take > place safely (e.g., no possible phrases following ", beer a" would give > lower cost than "<s> beer a"). This is indeed a risk-free strategy. > 2. Moses only checks the trailing words in the _current_ "hypotheses" when > deciding to recombine and doesn't look at previous hypotheses. So, 62 would > recombine with 47 because they both end with "a", regardless of what 17 and > 18 end in. > 3. Moses only checks at most the last two words in each hypothesis when > trying to recombine, regardless of what order of language model is used. > 4. Something else? > > Thanks! > Kevin > > >
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