To paint you a picture:
Imagine you have a rat in a labyrinth (the labyrinth is the TM and the search space). That rat is quite good at finding the center of that labyrinth. Now you somehow disable that rat's sense of smell, sense of direction, and long-term short-term memory (that's the LM). Can you expect the rat to find the center? Or will it just tumble around, bumping into walls and not find anything? That's what you did to the decoder when disabling the LM. Now you prune the TM. In the labyrinth that's like closing all the doors that would lead the rat away from the center. There are still a few corridors left, but they all point into the general direction of the point where the rat is supposed to go. Although it may never quite reach it. Now you put that same handicapped rat into the labyrinth where all ways lead more or less to the center. Are you really surprised that the clueless rat find the center nearly every time now? That's what happend. It's not a bug. The LM is probably the strongest feature in a MT system. If you take that away you see what happens. W dniu 2015-06-17 16:22, Read, James C napisał(a): > All I did was break the link to the language model and then perform > filtering. How is that a methodoligical mistake? How else would one test the > efficacy of the TM in isolation? > > I remain convinced that this is undersirable behaviour and therefore a bug. > > James > > ------------------------- > > FROM: Marcin Junczys-Dowmunt <junc...@amu.edu.pl> > SENT: Wednesday, June 17, 2015 5:12 PM > TO: Read, James C > CC: Arnold, Doug; moses-support@mit.edu > SUBJECT: Re: [Moses-support] Major bug found in Moses > > Hi James > > No, not at all. I would say that is expected behaviour. It's how search > spaces and optimization works. If anything these are methodological mistakes > on your side, sorry. You are doing weird thinds to the decoder and then you > are surprised to get weird results from it. > > W dniu 2015-06-17 16:07, Read, James C napisał(a): > > So, do we agree that this is undersirable behaviour and therefore a bug? > > James > > ------------------------- > > FROM: Marcin Junczys-Dowmunt <junc...@amu.edu.pl> > SENT: Wednesday, June 17, 2015 5:01 PM > TO: Read, James C > SUBJECT: Re: [Moses-support] Major bug found in Moses > > As I said. With an unpruned phrase table and an decoder that just optmizes > some unreasonble set of weights all bets are off, so if you get very low BLEU > point there, it's not surprising. It's probably jumping around in a very > weird search space. With a pruned phrase table you restrict the search space > VERY strongly. Nearly everything that will be produced is a half-decent > translation. So yes, I can imagine that would happen. > > Marcin > > W dniu 2015-06-17 15:56, Read, James C napisał(a): > > You would expect an improvement of 37 BLEU points? > > James > > ------------------------- > > FROM: Marcin Junczys-Dowmunt <junc...@amu.edu.pl> > SENT: Wednesday, June 17, 2015 4:32 PM > TO: Read, James C > CC: Moses-support@mit.edu; Arnold, Doug > SUBJECT: Re: [Moses-support] Major bug found in Moses > > Hi James, > > there are many more factors involved than just probability, for instance word > penalties, phrase penalities etc. To be able to validate your own claim you > would need to set weights for all those non-probabilities to zero. Otherwise > there is no hope that moses will produce anything similar to the most > probable translation. And based on that there is no surprise that there may > be different translations. A pruned phrase table will produce naturally less > noise, so I would say the behaviour you describe is quite exactly what I > would expect to happen. > > Best, > > Marcin > > W dniu 2015-06-17 15:26, Read, James C napisał(a): > > Hi all, > > I tried unsuccessfully to publish experiments showing this bug in Moses > behaviour. As a result I have lost interest in attempting to have my work > published. Nonetheless I think you all should be aware of an anomaly in > Moses' behaviour which I have thoroughly exposed and should be easy enough > for you to reproduce. > > As I understand it the TM logic of Moses should select the most likely > translations according to the TM. I would therefore expect a run of Moses > with no LM to find sentences which are the most likely or at least close to > the most likely according to the TM. > > To test this behaviour I performed two runs of Moses. One with an unfiltered > phrase table the other with a filtered phrase table which left only the most > likely phrase pair for each source language phrase. The results were truly > startling. I observed huge differences in BLEU score. The filtered phrase > tables produced much higher BLEU scores. The beam size used was the default > width of 100. I would not have been surprised in the differences in BLEU > scores where minimal but they were quite high. > > I have been unable to find a logical explanation for this behaviour other > than to conclude that there must be some kind of bug in Moses which causes a > TM only run of Moses to perform poorly in finding the most likely > translations according to the TM when there are less likely phrase pairs > included in the race. > > I hope this information will be useful to the Moses community and that the > cause of the behaviour can be found and rectified. > > James > > _______________________________________________ > Moses-support mailing list > Moses-support@mit.edu > http://mailman.mit.edu/mailman/listinfo/moses-support [1] Links: ------ [1] http://mailman.mit.edu/mailman/listinfo/moses-support
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