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--- courses:rg:overcoming_vocabulary_sparsity_in_mt_using_lattices [2010/11/29 23:32]
ivanova
+++ courses:rg:overcoming_vocabulary_sparsity_in_mt_using_lattices [2010/11/29 23:49]
ivanova
@@ Line 16: / Line 16: @@
 The article is about overcoming the problem of vocabulary sparsity in SMT. The sparsity occurs because many words can have inflection or can take different affixes while in the vocabulary we might not find all those forms.
 The authors of the article introduce three problems and their methods to overcome this challenges:
-(1) common stems are fragmented into many different forms in training data;
-(2) rare and unknown words are frequent in test data;
-(3) spelling variation creates additional sparseness problems.
-To solve the indicated problems authors modify training and test aligned bilingual data. The strong side of proposed approaches is that these techniques work for the large training data
+. common stems are fragmented into many different forms in training data;
+. rare and unknown words are frequent in test data;
+. spelling variation creates additional sparseness problems.
-===== Challenge (1) =====
+To solve the indicated problems authors modify training and test aligned bilingual data.
+The strong side of proposed approaches is that these techniques work for the large training data.
+===== Challenge 1 =====
 For the first challenge of vocabulary sparsity they don't intend to do complex morphological analysis, but they apply lightweight technique.
@@ Line 36: / Line 39: @@
 A Confusion Network (CN), also known as a sausage, is a weighted directed graph with the peculiarity that each path from the start node to the end node goes through all the other nodes. Each edge is labeled with a word and a (posterior) probability.
-Also we think that lattices may produce additional errors  so it would be good to have two experiments:
+Also we think that lattices may produce additional errors and it is computationally more expensive to work with lattices than with plain strings, so it would be good to have two experiments: one with lattices and one without lattices and compare the results.
-one with lattices and one without lattices and compare the results.
-===== Challenge (2) =====
+It is not really clear why they don't use lemmatizer instead of splitting. The amount of rules they might need for dealing with all the affixes, along with w- prefix, might be the same as if they wrote lemmatizer.
+===== Challenge 2 =====
 To translate rare and unknown words that are not in the dictionary the authors use 193 hand-written linguistic rules about how to cut-off affixes and get rid of inflection. The word that we get after cutting off the affix, might be in the dictionary, if not, algorithm will try to apply more rules to get a word that is in the dictionary.
-There is no information in the article about how the rule is selected in case there are suitable rules for one affix. Probably they have uniform distribution of rules and they leave to a language model to choose one.
+There is no information in the article about how the rule is selected in case there are several suitable rules for one affix. Probably they have uniform distribution of rules and they leave to a language model to choose one.
-===== Challenge (3) =====
-The third challenge is to correct spelling mistakes. If the word has one spelling mistakes, they try to correct. But they don't remove the original word, they just add the found options. If the word has more than one spelling mistakes, they do not deal with it.
+===== Challenge 3 =====
+The third challenge is to correct spelling mistakes. If the word has one spelling mistake, they try to correct it. But they don't remove the original word, they just add the found options. If the word has more than one spelling mistake, they do not deal with it.
 It is not clear from the article how exactly they correct the mistakes, for example
 mHAd__t__At - mHAd__v__At
-Do they have rules that for example probability of substituting __t__ by __v__ is bigger, than probability of substituting __t__ by __a__ ?
+It might be that they have rules that for example probability of substituting __t__ by __v__ is bigger, than probability of substituting __t__ by __a__.
 ===== Evaluation =====
+Typo-correct lower BLEU score on news wire development set is 54.4 which is lower than baseline in Table 5.  So it would be better to add additional evaluation line: all features without typo correction. This could show if typo-correction really helps to improve the final result. Or at least they should have provided human evaluation, e.g. although the BLEU score for the sentence without typo correction is the same as for the same sentence with typo-correction, the sentence looks better for a human when the typo is corrected.
+They aligned their data using LEAF alignment method. We discussed if it was possible to make the same alignment with GIZA++ but came to a conclusion that it is not.
 ===== Future work =====
 They can work with prefixes b-, l-, Al- and k- using similar approach as for w- prefix.
 They can look at the context for spelling correction.

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