Machine Translation Weekly 56: Beam Search and Models' Surprisal
Last year an EMNLP paper “On NMT Search Errors and Model Errors: Cat Got Your Tongue?” (that I discussed in MT Weekly 20) showed a mindblowing property of neural machine translation models that the most probable target sentence is not necessarily the best target sentence.
In NMT, we model the target sentence probably that is factorized using the chain rule into conditional token probabilities. We can imagine the target sentence generation like this: The model estimates the probability of the first word given the source sentence. From this distribution, we pick one word. The model then estimates the probability of the second word given the first word and the source sentence. We select the second word from this distribution, and so on…
Previously, we thought that exact inference under this factorization is intractable, but we can approximate the inference using the beam search algorithm. The exact inference would mean that we would keep all possibilities in each step instead of picking one word (that I suggested in the previous paragraph). The problem is that the number of possible target sequences grows exponentially with the length and the number of possible 20word target sequences is comparable to the number of atoms in the known universe. Therefore, in practice, we use an algorithm called beam search. In this algorithm, we only keep a small number of “surviving” hypotheses after every time step.
The last year’s paper presented a clever way of doing the exact inference (that is still too slow for any practical use, but fast enough for experimenting) and found out that often, the most probable target sentence is an empty string. In other words, the beam search does not approximate the exact search. In fact, search errors are necessary to find a good translation using a welltrained model. An additional trick that we use in the beam search is we add length normalization, i.e., boost the probability of longer sequences, so highprobability too short sentences fell out of the beam during decoding. This heuristics (that does not have a theoretical justification) helps also with the exact inference, but beam search still gets better translation quality.
A paper that will be published at his year’s EMNLP (If Beam Search is the Answer, What was the Question?) suggests an alternative decoding objective that says that the target sentence should be both highly probable and at the same minimally surprising in terms of information theory.
The paper provides three types of arguments that should support this objective:

mathematical,

cognitive motivation, and

experimental.
To be honest, I do not entirely buy any of these arguments, but together, they suggest that this might be a way to go.
The mathematical argument is that optimizing the surprisal of the target sentence given the model is equivalent to doing beam search. They show formal proof of this; however, there are, in fact, infinitely many alternative formulations (e.g., not using the surprisal, but directly the probabilities) that would have the same theoretical property.
The second argument is that it corresponds to the uniform information density theory. It is a psycholinguistic theory that says that people prefer such sentences where the information is uniformly distributed. Minimizing the surprisal during decoding ensures this uniform information density with respect to the translation model. The entropies with respect to standard language models are known to highly correlate with human surprisal. However, machine translation models are conditional language models, and as far as I know, there is no study that would say MT models have the same properties as language models in this matter.
Finally, they do experiments with the modified decoding objective: they combine the probability assigned by the model with regularization terms that ensure the uniform distribution of information. The translation quality is basically the same as when using the length normalization. However, in their experiments, the translation quality of the lengthnormalized beam search decreases with the increasing beam size, which according to my experience means that they did not properly tune the weight of the length normalization and the length normalization might be in fact better than the proposed regularization.
When I read the paper about the search errors last year, I was not thinking that the training objective is fine; we only want something else than the most probable target sequence. I always thought that the ultimate solution is finding a different formalization of the target sentence probability, so this paper showed this problem from a perspective that was quite new to me.
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@misc{libovicky2020blog1025,
author = "Jindřich Libovický",
title = "Jindřich's Blog  Machine Translation Weekly 56: Beam Search and Models' Surprisal",
year = "2020",
month = oct,
url = "https://jlibovicky.github.io/2020/10/25/MTWeeklyBeachSearchandSurprisal",
note = "Online, Accessed: 29.11. 2020"
}