Title: Unsupervised Machine Translation Using Monolingual Corpora Only
Authors: Guillaume Lample
Alexis Conneau
Ludovic Denoyer
Marc'Aurelio Ranzato
Venue: ICLR 2018
Keywords: unsupervised
machine translation
adversarial

Abstract: Machine translation has recently achieved impressive performance thanks to recent advances in deep learning and the availability of large-scale parallel corpora. There have been numerous attempts to extend these successes to low-resource language pairs, yet requiring tens of thousands of parallel sentences. In this work, we take this research direction to the extreme and investigate whether it is possible to learn to translate even without any parallel data. We propose a model that takes sentences from monolingual corpora in two different languages and maps them into the same latent space. By learning to reconstruct in both languages from this shared feature space, the model effectively learns to translate without using any labeled data. We demonstrate our model on two widely used datasets and two language pairs, reporting BLEU scores of 32.8 and 15.1 on the Multi30k and WMT English-French datasets, without using even a single parallel sentence at training time.

TL;DR: We propose a new unsupervised machine translation model that can learn without using parallel corpora; experimental results show impressive performance on multiple corpora and pairs of languages.

Review discussion:
This paper describes an approach to train a neural machine translation system without parallel data. Starting from a word-to-word translation lexicon, which was also learned with unsupervised methods, this approach combines a denoising auto-encoder objective with a back-translation objective, both in two translation directions, with an adversarial objective that attempts to fool a discriminator that detects the source language of an encoded sentence. These five objectives together are sufficient to achieve impressive English <-> German and Engish <-> French results in Multi30k, a bilingual image caption scenario with short simple sentences, and to achieve a strong start for a standard WMT scenario.
This is very nice work, and I have very little to criticize. The approach is both technically interesting, and thorough in that it explores and combines a host of ideas that could work in this space (initial bilingual embeddings, back translation, auto-encoding, and adversarial techniques). And it is genuinely impressive to see all these pieces come together into something that translates substantially better than a word-to-word baseline.
But the aspect I like most about this paper is the experimental analysis. Considering that this is a big, complicated system, it is crucial that the authors included both an ablation experiment to see which pieces were most important, and an experiment that indicates the amount of labeled data that would be required to achieve the same results with a supervised system.
In terms of specific criticisms:
In Equations (2), consider replacing C(y) with C(M(x)), or use compose notation, in order to make x-hat's relationship to x clear and self-contained within the equation.
I am glad you take the time to give your model selection criterion it's own section in 3.2, as it does seem to be an important part of this puzzle.
However, it would be nice to provide actual correlation statistics rather than an anecdotal illustration of correlation.
In the first paragraph of Section 4.5, I disagree with the sentence, "Similar observations can be made for the other language pairs we considered." In fact, I would go so far as to say that the English to French scenario described in that paragraph is a notable outlier, in that it is the other language pair where you beat the oracle re-ordering baseline in both Multi30k and WMT.
When citing Shen et al., 2017, consider also mentioning the following:
Controllable Invariance through Adversarial Feature Learning; Qizhe Xie, Zihang Dai, Yulun Du, Eduard Hovy, Graham Neubig; NIPS 2017; https://arxiv.org/abs/1705.11122
Response read -- thanks.
We thank the reviewer for the feedback and comments.
We clarified Equation (2), and also provided a correlation score between the unsupervised criterion and the actual test performance, thanks for the suggestion. The paper now contains: “The unsupervised model selection criterion is used both to a) determine when to stop training and b) to select the best hyper-parameter setting across different experiments. In the former case, the Spearman correlation coefficient between the proposed criterion and BLEU on the test set is 0.95 in average. In the latter case, the coefficient is in average 0.75, which is fine but not nearly as good.
For instance, the BLEU score on the test set of models selected with the unsupervised criterion are sometimes up to 1 or 2 BLEU points below the score of models selected using a small validation set of 500 parallel sentences.”
The work of Xie et al. was indeed relevant and we added it as a citation in the related work section of the updated version.
As for the first paragraph of Section 4.5, we will clarify that “similar observations” refer to improvements as we iterate. Thank you for pointing this out.
The authors present an approach for unsupervised MT which uses a weighted loss function containing 3 components: (i) self reconstruction (ii) cross reconstruction and (iii) adversarial loss. The results are interesting (but perhaps less interesting than what is hinted in the abstract).
1) In the abstract the authors mention that they achieve a BLEU score of 32.8 but omit the fact that this is only on Multi30K dataset and not on the more standard WMT datasets. At first glance, most people from the field would assume that this is on the WMT dataset. I request the authors to explicitly mention this in the abstract itself (there is clearly space and I don't see why this should be omitted)
2) In section 2.3, the authors talk about the Noise Model which is inspired by the standard Denoising Autoencoder setup.
While I understand the robustness argument in the case of AEs I am not convinced that the same applies to languages. Such random permutations can often completely alter the meaning of the sentence. The ablation test seems to suggest that this process helps. I read another paper which suggests that this noise does not help (which intuitively makes sense). I would like the authors to comment on this (of course, I am not asking you to compare with  the other paper but I am just saying that I have read contradicting observations - one which seems intuitive and the other does not).
3) How were the 3 lambdas in Equation 3 selected ? What ranges did you consider. The three loss terms seem to have very different ranges. How did you account for that?
4) Clarification: In section 2.5 what exactly do you mean by "as long as the two monolingual corpora exhibit strong structure in feature space." How do you quantify this ?
5) In section 4.1, can you please mention the exact number of sentences that you sampled from WMT'14. You mention that selected sentences from 15M random pairs but how many did you select ? The caption of one of the figure mentions that there were 10M sentences. Just want to confirm this.
6) The improvements are much better on the Multi30k dataset. I guess this is because this dataset has smaller sentences with smaller vocabulary. Can you provide a table comparing the average number of sentences and vocabulary size of the two datasets (Multi30k and WMT).
7) The ablation results are provided only for the Multi30k dataset. Can you provide similar results for the WMT dataset. Perhaps this would help in answering my query in point (2) above.
8) Can you also check the performance of a PBSMT system trained on 100K parallel sentences?
Although NMT outperforms PBSMT when the data size is large, PBSMT might still be better suited for low resource settings.
9) There are some missing citations (already pointed by others in the forum) . Please add those.
I have noted the clarifications posted by the authors. I still have concerns about a couple of things.
For example, I am still not convinced about the justification given for word order. I understand that empirically it works better but I don't get the intuition.
Similarly, I don't get the argument about "strong structure in feature space". This is just a conjecture and it is very hard to measure it. I would request the authors to not emphasize on it or give a different more grounded intuition.
I do acknowledge the efforts put in by the authors to address some of my comments and for that I would like to change my rating a bit.
We thank the reviewer for the feedback and comments. We address each of them in turn:
1) It is true that the 32.8 BLEU score in the abstract was misleading, thank you for pointing this out. We updated the paper as follows: “We demonstrate our model on two widely used datasets and two language pairs, reporting BLEU scores of 32.8 and 15.1 on the Multi30k and WMT English-French datasets, without using even a single parallel sentence at training time.”
2) Modifying word orders and dropping words definitely alters the meaning of the sentence.
However, without this component, we observed that the autoencoder was simply learning to copy the words in the input sentence one-by-one. Adding noise to the input sentence turned out to be an efficient solution that prevents the model from converging to that trivial solution.
It is true that in some other tasks like sentence classification or machine translation, adding noise to the input sentence might alter the meaning of the sentence and deteriorate the overall performance of the system, but in the case of auto-encoding, this turned out to be necessary for us.
In particular, on WMT en-fr / fr-en we obtain 5.24 / 6.57 without word shuffling (but with word dropout), and 1.69 / 5.54 when not using any form of noise. Please see response to 7) below for more results on the ablation study for WMT.
3) We also expected the tuning of these coefficients to be critical. We ran a few experiments with values from 1e-5 to 10, but in practice, we observed very small differences using different coefficients compared to fixing everything to 1.
4) “as long as the two monolingual corpora exhibit strong structure in feature space." This sentence was indeed incorrect, thank you for spotting this. We corrected it to “as long as the two latent representations exhibit strong structure in feature space”. What we meant is that if the learned word embeddings were iid distributed, then we would not be able to align them as any rotation would yield an equivalent matching of the two distributions. The reason why we can align well is because there are asymmetries which the algorithm exploits to align the two spaces. We are not aware of any study quantifying the structure in embedding space to the quality of the alignment. Here, we just meant to provide an intuition for our approach.
5) Thanks for noticing this, there was indeed a mistake in the caption. The unsupervised method uses 15M sentences (each for French and English, so 30M total) with WMT14 en-fr, and 3.6M sentences with WMT16 de-en (so 1.8M for each language).
6) We provided some statistics about the vocabulary size, and the number of monolingual sentences both for WMT and MMT1, in Table 1 of the updated version of the paper.
7) The experimental setup used in this paper is quite expensive, this is why we initially considered the much smaller MMT1 dataset to perform the ablations study, and to use that to decide on the best parameters for WMT. We ran new experiments to study the impact of each component when training on the WMT dataset, for the en-fr and fr-en language pairs.
In particular, we obtain on en-fr / fr-en:
- Without word shuffle, but with word dropout: 5.24 / 6.57
- Without word shuffle, and without word dropout: 1.69 / 5.54
- Without adversarial training: 10.45 / 10.29
- Without auto-encoding: 1.20 / 1.21
- Without word embeddings pretraining: 11.11 / 10.86
- Without word embeddings, and without cross-domain training: 1.44 / 1.30
- Without cross-domain training: 2.65 / 2.44
8) We ran experiments with a phrase-based system and obtained much better results than with a standard NMT system. Actually, our phrase-based models with 10k parallel sentences obtained 15.5 and 16.0 BLEU, which is roughly on par with what we report in the paper with our NMT model for 100k pairs. Note that our supervised NMT baseline in the paper is a bit weak, as we set a very large minimum word-count cutoff to reduce the vocabulary size and accelerate the experiments (and our unsupervised approach suffers from the same issue). We are currently running further experiments for other parallel corpora sizes and the de-en pair, and will report results with PBSMT in the paper very soon. Thank you for your suggesting this, these results are significantly better than what we expected, this will be very valuable to the paper, and opens new research directions.
9) We added the relevant citations as suggested in the comments.
This paper introduces an architecture for training a MT model without any parallel material, and tests it on benchmark datasets (WMT and captions) for two language pairs.
Although the resulting performance is only about half that of a more traditional model, the fact that this is possible at all is remarkable.
The method relies on fairly standard components which will be familiar to most readers: a denoising auto-encoder and an adversarial discriminator. Not much detail is given on the actual models used, for which the authors mainly refer to prior work. This is disappointing: the article would be more self-contained by providing even a high-level description of the models, such as provided (much too late) for the discriminator architecture.
Misc comments:
"domain" seems to be used interchangeably with "language". This is unfortunate as "domain" has another, specific meaning in NLP in general and SMT in partiular. Is this intentional (if so what is the intention?) or is this just a carry-over from other work in cross-domain learning?
Section 2.3: How do you sample permutations for the noise model, with the constraint on reordering range, in the general case of sentences of arbitrary lengths?
Section 2.5: "the previously introduced loss [...] mitigates this concern" -- How? Is there a reference backing this?
Figure 3: In the caption, what is meant by "(t) = 1"? Are these epochs only for the first iteration (from M(1) to M(2))?
Section 4.1: Care is taken to avoid sampling corresponding src and tgt sentences.
However, was the parallel corpus checked for duplicates or near duplicates? If not, "aligned" segments may still be present. (Although it is clear that this information is not used in the algorithm)
This yields a natural question: Although the two monolingual sets extracted from the parallel data are not aligned, they are still very close. It would be interesting to check how the method behaves on really comparable corpora where its advantage would be much clearer.
Section 4.2 and Table 1: Is the supervised learning approach trained on the full parallel corpus? On a parallel corpus of similar size?
Section 4.3: What are the quoted accuracies (84.48% and 77.29%) measured on?
Section 4.5: Experimental results show a regular inprovement from iteration 1 to 2, and 2 to 3. Why not keep improving performance? Is the issue training time?
References: (He, 2016a/b) are duplicates
Response read -- thanks.
We thank the reviewer for the feedback and comments.
We did not provide details about the architecture mainly because of lack of space, but we will add it in the updated version of the paper.
Briefly, our architecture was composed of a standard encoder-decoder, with 3 LSTM layers, and an attention model without input-feeding. The embedding and LSTM hidden dimensions were set to 300.
We now address the comments in turn:
- We casted machine translation in the unsupervised setting as the problem to match distributions of latent features, which can be seen as a particular instance of domain adaptation where “domain” refers to a particular language. We will make sure to clarify this in the next version of the paper.
- To generate random permutations with the reordering range constraint, we generate a random vector of the size of the sentence, and sort it by indexes. In NumPy, for a sentence of size n, it will look like: `  x = (np.arange(n) + alpha * np.random.rand(n)).argsort()'` Where alpha is a tunable parameter. alpha = 0 implies that x is the identity, alpha = infinity can return any permutation. With alpha = 4, for all i in [0, n[ we have |x[i] - i| <= 3. This has been added in the section 2.3 of the paper.
- "the previously introduced loss [...] mitigates this concern": we are not aware of any reference about this, but this is the intuition we had while designing our loss function. The intuition is that auto-encoding with adversarial training ensures that latent representations of sentences in the two languages have similar distributions, but nothing constrains the system to actually translate (e.g., the sentence “je parle français” could be mapped into a latent space which could be decoded into the English sentence “the car is red”, which is a correct English sentence but not a good translation).
However, the back-translation term does make sure that the latent representations actually (and eventually) correspond to each other, as the system has to produce a ground truth translation from a noisy source (and the auto-encoding term helps mapping noisy sentences into the same latent representation).
- In the caption of Figure 3, “(t) = 1” indeed represents the training from M(1) to M(2). We clarified this in the updated version of the paper.
- We did some experiments to investigate whether some duplicates of the removed sentences might be present among the selected sentences. To do so, we used a simple technique based on weighted bag-of-words embeddings, to retrieve the most similar sentences, and overall we have not been able to find very good matching pairs. We concluded that our two selected set of sentences were different enough in the sense that most sentences will not have an equivalent translation in the opposite language.
However, it is true that the two domains remain similar. We are planning to investigate on the impact of the similarity of the two monolingual corpora on the translation quality in the future.
- The supervised learning approach is trained on the full corpora (both for Multi30k and WMT).
- The accuracies are measured on the word translation retrieval: given a test dictionary of 5000 pairs of words, we estimate how frequently a source word is properly mapped to its associated target word.
- We ran a fourth iteration on Multi30k, but did not observe any improvement. The results would have improved by about 0.5/1 BLEU point if our unsupervised criterion had been perfect (see response to reviewer 1 about the quality of the criterion).
However, using our criterion, this fourth iteration gave the same BLEU as the third iteration.
We would like to thank the reviewers for all their comments and constructive feedback. We answered to each review individually, and uploaded a revision of the paper. Here is brief summary of the changes we made:
- updated some of the claims in the paper
- added details about the results in the abstract
- provided some statistics about the correlation between our unsupervised criterion and the BLEU test score
- added result with phrase based baseline
- added details about the model architecture
- explained how we generate random permutations with a constraint on the reordering
- simplified some notation
- clarified a few sentences / fixed typos
- added a table to compare the average number of sentences and the vocabulary size in the considered datasets
- added missing citations

Meta-review:
role: AC
data: comment: Authors, 

Could I ask you to respond to the reviewers for discussion? While the reviewers here are quite positive, there are some points of clarification and concerns that would be nice to hash out. 
role: PC
data: comment: This work presents some of the first results on unsupervised neural machine translation. The group of reviewers is highly knowledgeable in machine translation, and they were generally very impressed by the results and the think it warrants a whole new area of research noting "the fact that this is possible at all is remarkable.". There were some concerns with the clarity of the details presented and how it might be reproduced, but it seems like much of this was cleared up in the discussion. The reviewers generally praise the thoroughness of the method, the experimental clarity, and use of ablations. One reviewer was less impressed, and felt more comparison should be done.

Decision: Accept (Poster)