Back in 2016, one of the trendy topics was reinforcement learning and other forms of optimizing NMT directly towards some more relevant metrics rather than using cross-entropy of the conditional word distributions. Standard machine translation models are trained to maximize single-word conditional distribution, which is not exactly what we are interested in when doing machine translation. What we really want is good translation quality, which does not factorize over words (or subwords), and can only be assessed on the sequence level. This line of thinking led to research on reinforcement learning in machine translation which claimed some successes but after its popularity peak it seems to me it kind of died out.

Recently, I came across a paper that critically evaluates these approaches and explains what might be wrong with reinforcement learning in machine translation. The paper is called On the Weaknesses of Reinforcement Learning for Neural Machine Translation, it comes from the Hebrew University of Jerusalem and will be presented at this year’s ICRL.

Normally, machine translation models are trained word by word. We feed the model with a prefix of a reference target sentence, and the model guesses what the next word is. Based on how incorrect the guess was, the model gets updated. Reinforcement learning works differently. It is a type of machine learning that you would expect in robotics: an agent tries to do something on its own and based on how much it succeeds, it changes its behavior for the next attempts. When applied to machine translation, we let the model generate an entire sentence (without providing the reference sentence prefix), measure how good it is (i.e., a reward) and based on that update the model. The tricky thing we need to deal with is that we do not really know what words contributed to the reward.

The methods include minimum risk training that estimates the expected BLEU score, various adaptations of the REINFORCE equation, and using a generator-discriminator setup to learn more adequate reward function than BLEU (or another score). One thing all the papers had in common was that they always started with an already-trained model and only fine-tuned the model to get a slightly higher translation quality.

This paper I mentioned earlier is very skeptical about the methods. Although they are always well theoretically justified, the empirical improvement of the translation quality is small and might have a different reason. The paper presents experiments that show that most reinforcement learning approaches do not teach the model anything new it did not know before. It makes the output distribution peakier, so if the model was already good enough, it just reinforces it in its good decisions (which presumably helps in beam search). However, in the experiments, they never observe that a correct (and thus reward-worth) word that scored really badly in the model got ranked higher after the finetuning.

I always thought that the reason why these methods do not generate much improvement was that sentence-level machine translation evaluation is a hard problem. Even though there exist few decent methods, most of them are too slow to be used in a setup when the millions of training sentences need to be repeatedly evaluated during training. Anyway, this paper suggests that even if we had such an evaluation metric, it probably would not help much.

Maybe the reason why reinforcement learning does not work as well as we would expect is the assumption that a trained NMT model describes a good distribution over target sequences. From a paper that I discussed in MT Weekly 20, we know that this is not entirely true. I would say that by pure luck, the beam search algorithm leads to good translations, even though its outputs are not the sentences that are the most probable given the model. Maybe if we can fix the probability, reinforcement learning would become much more useful.

@article{choshen2019weaknesses,
title={On the Weaknesses of Reinforcement Learning for Neural Machine Translation},
author={Choshen, Leshem and Fox, Lior and Aizenbud, Zohar and Abend, Omri},
journal={arXiv preprint arXiv:1907.01752},
year={2019}
}