Mimicking Word Embeddings using Subword RNNs

Word embeddings improve generalization over lexical features by placing each word in a lower-dimensional space, using distributional information obtained from unlabeled data. However, the effectiveness of word embeddings for downstream NLP tasks is limited by out-of-vocabulary (OOV) words, for which embeddings do not exist. In this paper, we present MIMICK, an approach to generating OOV word embeddings compositionally, by learning a function from spellings to distributional embeddings. Unlike prior work, MIMICK does not require re-training on the original word embedding corpus; instead, learning is performed at the type level. Intrinsic and extrinsic evaluations demonstrate the power of this simple approach. On 23 languages, MIMICK improves performance over a word-based baseline for tagging part-of-speech and morphosyntactic attributes. It is competitive with (and complementary to) a supervised character-based model in low-resource settings.Comment: EMNLP 201

Think Globally, Embed Locally - Locally Linear Meta-embedding of Words

Distributed word embeddings have shown superior performances in numerous Natural Language Processing (NLP) tasks. However, their performances vary significantly across different tasks, implying that the word embeddings learnt by those methods capture complementary aspects of lexical semantics. Therefore, we believe that it is important to combine the existing word embeddings to produce more accurate and complete \emph{meta-embeddings} of words. For this purpose, we propose an unsupervised locally linear meta-embedding learning method that takes pre-trained word embeddings as the input, and produces more accurate meta embeddings. Unlike previously proposed meta-embedding learning methods that learn a global projection over all words in a vocabulary, our proposed method is sensitive to the differences in local neighbourhoods of the individual source word embeddings. Moreover, we show that vector concatenation, a previously proposed highly competitive baseline approach for integrating word embeddings, can be derived as a special case of the proposed method. Experimental results on semantic similarity, word analogy, relation classification, and short-text classification tasks show that our meta-embeddings to significantly outperform prior methods in several benchmark datasets, establishing a new state of the art for meta-embeddings

Evaluating Word Embeddings in Multi-label Classification Using Fine-grained Name Typing

Embedding models typically associate each word with a single real-valued vector, representing its different properties. Evaluation methods, therefore, need to analyze the accuracy and completeness of these properties in embeddings. This requires fine-grained analysis of embedding subspaces. Multi-label classification is an appropriate way to do so. We propose a new evaluation method for word embeddings based on multi-label classification given a word embedding. The task we use is fine-grained name typing: given a large corpus, find all types that a name can refer to based on the name embedding. Given the scale of entities in knowledge bases, we can build datasets for this task that are complementary to the current embedding evaluation datasets in: they are very large, contain fine-grained classes, and allow the direct evaluation of embeddings without confounding factors like sentence contextComment: 6 pages, The 3rd Workshop on Representation Learning for NLP (RepL4NLP @ ACL2018

Distilling word vectors from contextualised language models

Although contextualised language models (CLMs) have reduced the need for word embedding in various NLP tasks, static representations of word meaning remain crucial in tasks where words have to be encoded without context. Such tasks arise in domains such as information retrieval. Compared to learning static word embeddings from scratch, distilling such representations from CLMs has advantages in downstream tasks[68],[2]. Usually, the embedding of a word w is distilled by feeding random sentences that mention w to a CLM and extracting the parameters. In this research, we assume distilling word embeddings from CLMs can be improved by feeding more informative mentions to a CLM. Therefore, as a first contribution in this thesis, we proposed a strategy for sentence selection by using a topic model. Since distilling high-quality word embeddings from CLMs requires many mentions for each word, we investigate whether we can obtain decent word embeddings by using a few but carefully selected mentions of each word. As our second contribution, we explored a range of sentence selection strategies and tested their generated word embeddings on various evaluation tasks. We found that 20 informative sentences per word are sufficient to obtain competitive word embeddings, especially when the sentences are selected by our proposed strategies. Besides improving the sentence selection strategy, as our third contribution, we also studied other strategies for obtaining word embeddings. We found that SBERT embeddings capture an aspect of word meaning that is highly complementary to the mention embeddings we previously focused on. Therefore, we proposed combining the vectors generated from these two methods through a contrastive learning model. The results confirm that combining these vectors leads to more informative word embeddings. In conclusion, this thesis shows that better static word embeddings can be efficiently distilled from CLMs by strategically selecting sentences and combining complementary method

Knowledge-aware Complementary Product Representation Learning

Learning product representations that reflect complementary relationship plays a central role in e-commerce recommender system. In the absence of the product relationships graph, which existing methods rely on, there is a need to detect the complementary relationships directly from noisy and sparse customer purchase activities. Furthermore, unlike simple relationships such as similarity, complementariness is asymmetric and non-transitive. Standard usage of representation learning emphasizes on only one set of embedding, which is problematic for modelling such properties of complementariness. We propose using knowledge-aware learning with dual product embedding to solve the above challenges. We encode contextual knowledge into product representation by multi-task learning, to alleviate the sparsity issue. By explicitly modelling with user bias terms, we separate the noise of customer-specific preferences from the complementariness. Furthermore, we adopt the dual embedding framework to capture the intrinsic properties of complementariness and provide geometric interpretation motivated by the classic separating hyperplane theory. Finally, we propose a Bayesian network structure that unifies all the components, which also concludes several popular models as special cases. The proposed method compares favourably to state-of-art methods, in downstream classification and recommendation tasks. We also develop an implementation that scales efficiently to a dataset with millions of items and customers