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Title: Cross-Lingual Ability of Multilingual BERT: An Empirical Study

Paper content:
Recent work has exhibited the surprising cross-lingual abilities of multilingual BERT (M-BERT) -- surprising since it is trained without any cross-lingual objective and with no aligned data.
In this work, we provide a comprehensive study of the contribution of different components in M-BERT to its cross-lingual ability.
We study the impact of linguistic properties of the languages, the architecture of the model, and of the learning objectives.
The experimental study is done in the context of three typologically different languages -- Spanish, Hindi, and Russian -- and using two conceptually different NLP tasks, textual entailment and named entity recognition.
Among our key conclusions is the fact that lexical overlap between languages plays a negligible role in the cross-lingual success, while the depth of the network is an important part of it
Embeddings of natural language text via unsupervised learning, coupled with sufficient supervised training data, have been ubiquitous in NLP in recent years and have shown success in a wide range of monolingual NLP tasks, mostly in English.
Training models for other languages have been shown more difficult, and recent approaches relied on bilingual embeddings that allowed the transfer of supervision in high resource languages like English to models in lower resource languages; however, inducing these bilingual embeddings required some level of supervision (Upadhyay et al., 2016) .
Multilingual BERT 1 (M-BERT), a Transformer-based (Vaswani et al., 2017) language model trained on raw Wikipedia text of 104 languages suggests an entirely different approach.
Not only the model is contextual, but its training also requires no supervision -no alignment between the languages is done.
Nevertheless, and despite being trained with no explicit cross-lingual objective, M-BERT produces a representation that seems to generalize well across languages for a variety of downstream tasks (Wu & Dredze, 2019) .
In this work, we attempt to develop an understanding of the success of M-BERT.
We study a range of aspects on a couple of different NLP tasks, in order to identify the key components in the success of the model.
Our study is done in the context of only two languages, source (typically English) and target (multiple, quite different languages).
By involving only a pair of languages, we can study the performance on a given target language, ensuring that it is influenced only by the cross-lingual transfer from the source language, without having to worry about a third language interfering.
We analyze the two-languages version of M-BERT (B-BERT, from now on) in three orthogonal dimensions:
(i) Linguistics properties and similarities of target and source languages;
(ii) Network Architecture, and
(iii) Input and Learning Objective.
One hypothesis that came up when the people thoughts about the success of M-BERT is due to some level of language similarity.
This could be lexical similarity (shared words or word-parts) or structural similarities, or both.
We, therefore, investigate the contribution of word-piece overlap -the extent to which the same word-pieces appear in both source and target languages -and distinguish it from other similarities, which we call structural similarity between the source and target languages.
Surprisingly, as we show, B-BERT is cross-lingual even when there is absolutely no word-piece overlap.
That is, other aspects of language similarity must be contributing to the cross-lingual capabilities of the model.
This is contrary to Pires et al. (2019) hypothesis that M-BERT gains its power from shared word-pieces.
Furthermore, we show that the amount of word-piece overlap in B-BERT's training data contributes little to performance improvements.
Our study of the model architecture addresses the importance of
(i) The network depth,
(ii) the number of attention heads, and
(iii) the total number of model parameters in B-BERT.
Our results suggest that depth and the total number of parameters of B-BERT are crucial for both monolingual and cross-lingual performance, whereas multi-head attention is not a significant factor -a single attention head B-BERT can already give satisfactory results.
To understand the role of the learning objective and the input representation, we study the effect of
(i) the next sentence prediction objective,
(ii) the language identifier in the training data, and
(iii) the level of tokenization in the input representation (character, word-piece, or word tokenization).
Our results indicate that the next sentence prediction objective actually hurts the performance of the model while identifying the language in the input does not affect B-BERT's performance crosslingually.
Our experiments also show that character-level and word-level tokenization of the input results in significantly worse performance than word-piece level tokenization.
Overall, we provide an extensive set of experiments on three source-target language pairs, EnglishSpanish, English-Russian, and English-Hindi.
We chose these target languages since they vary in scripts and typological features.
We evaluate the performance of B-BERT on two very different downstream tasks: cross-lingual Named Entity Recognition -a sequence prediction task the requires only local context -and cross-lingual Textual Entailment Dagan et al. (2013) that requires more global representation of the text.
Ours is not the first study of M-BERT.
(Wu & Dredze, 2019) and (Pires et al., 2019) identified the cross-lingual success of the model and tried to understand it.
The former by considering M-BERT layerwise, relating cross-lingual performance with the amount of shared word-pieces and the latter by considering the model's ability to transfer between languages as a function of word order similarity in languages.
However, both works treated M-BERT as a black box and compared M-BERT's performance on different languages.
This work, on the other hand, examines how B-BERT performs cross-lingually by probing its components, along multiple aspects.
We also note that some of the architectural conclusions have been observed earlier, if not investigated, in other contexts.
; Yang et al. (2019) argued that the next Sentence prediction objective of BERT (the monolingual model) is not very useful; we show that this is the case in the cross-lingual setting.
Voita et al. (2019) prunes attention heads for a transformer based machine translation model and argues that most attention heads are not important; in this work, we show that the number of attention heads is not important in the cross-lingual setting.
Our contributions are threefold:
(i) we provide the first extensive study of the aspects of the multilingual BERT that give rise to its cross-lingual ability.
(ii) We develop a methodology that facilitates the analysis of similarities between languages and their impact on cross-lingual models; we do this by mapping English to a Fake-English language, that is identical in all aspects to English but shares not word-pieces with any target language.
Finally,
(iii) we develop a set of insights into B-BERT, along linguistics, architectural, and learning dimensions, that would contribute to further understanding and to the development of more advanced cross-lingual neural models.
This paper provides a systematic empirical study addressing the cross-lingual ability of B-BERT.
The analysis presented here covers three dimensions: (1) Linguistics properties and similarities of the source and target languages, (2) Neural Architecture, and (3) Input representation and Learning Objective.
In order to gauge the language similarity aspect needed to make B-BERT successful, we created a new language -Fake-English -and this allows us to study the effect of word-piece overlap while maintaining all other properties of the source language.
Our experiments reveal some interesting and surprising results like the fact that word-piece overlap on the one hand, and multi-head attention on the other, are both not significant, whereas structural similarity and the depth of B-BERT are crucial for its cross-lingual ability.
While, in order to better control interference among languages, we studied the cross-lingual ability of B-BERT instead of those of M-BERT, it would be interesting now to extend this study, allowing for more interactions among languages.
We leave it to future work to study these interactions.
In particular, one important question is to understand the extent to which adding to M-BERT languages that are related to the target language, helps the model's cross-lingual ability.
We introduced the term Structural Similarity, despite its obscure definition, and show its significance in cross-lingual ability.
Another interesting future work could be to develop a better definition and, consequently, a finer set of experiments, to better understand the Structural similarity and study its individual components.
Finally, we note an interesting observation made in Table 8 .
We observe a drastic drop in the entailment performance of B-BERT when the premise and hypothesis are in different languages.
(This data was created using XNLI when in the original form the languages contain same premise and hypothesis pair).
One of the possible explanations could be that BERT is learning to make textual entailment decisions by matching words or phrases in the premise to those in the hypothesis.
This question, too, is left as a future direction.
In the main text, we defined structural similarity as all the properties of a language that is invariant to the script of the language, like morphology, word-ordering, word-frequency, etc..
Here, we analyze 2 sub-components of structural similarity -word-ordering similarity and word-frequency (Unigram frequency) similarity to understand the concept of structural similarity better.

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