Nelson F. Liu

Email: nfliu [strudel] cs.stanford.edu

Links: [Full CV] [Twitter] [Github] [Google Scholar] [Blog]

Recent News

• (10/2019) Worked with several other junior NLP PhD students to put together a blog post with student perspectives on the NLP PhD application process. Happy to talk to anyone applying, feel free to shoot me an email.
• (8/2019) Paper on Quoref, a new reading comprehension dataset with questions that require resolving coreference, accepted to EMNLP 2019.
• (8/2019) I'm participating in the Center for Brains, Minds, and Machines' summer course.

Publications

• Quoref: A Reading Comprehension Dataset with Questions Requiring Coreferential Reasoning
  Pradeep Dasigi, Nelson F. Liu, Ana Marasović, Noah A. Smith, and Matt Gardner.
  In Conference on Empirical Methods in Natural Language Processing & International Joint Conference on Natural Language Processing (EMNLP-IJCNLP), 2019.
  [bib] [abstract] [poster] [dataset] [leaderboard]

  Machine comprehension of texts longer than a single sentence often requires coreference resolution. However, most current reading comprehension benchmarks do not contain complex coreferential phenomena and hence fail to evaluate the ability of models to resolve coreference. We present a new crowdsourced dataset containing more than 24K span-selection questions that require resolving coreference among entities in over 4.7K English paragraphs from Wikipedia. Obtaining questions focused on such phenomena is challenging, because it is hard to avoid lexical cues that shortcut complex reasoning. We deal with this issue by using a strong baseline model as an adversary in the crowdsourcing loop, which helps crowdworkers avoid writing questions with exploitable surface cues. We show that state-of-the-art reading comprehen- sion models perform significantly worse than humans on this benchmark—the best model performance is 70.5 F1, while the estimated human performance is 93.4 F1.



• Barack's Wife Hillary: Using Knowledge Graphs for Fact-Aware Language Modeling
  Robert L. Logan IV, Nelson F. Liu, Matthew E. Peters, Matt Gardner, and Sameer Singh.
  In Annual Meeting of the Association for Computational Linguistics (ACL), 2019.
  [bib] [abstract] [poster] [code] [dataset]

  Modeling human language requires the ability to not only generate fluent text but also encode factual knowledge. However, traditional language models are only capable of remembering facts seen at training time, and often have difficulty recalling them. To address this, we introduce the knowledge graph language model (KGLM), a neural language model with mechanisms for selecting and copying facts from a knowledge graph that are relevant to the context. These mechanisms enable the model to render information it has never seen before, as well as generate out-of-vocabulary tokens. We also introduce the Linked WikiText-2 dataset, a corpus of annotated text aligned to the Wikidata knowledge graph whose contents (roughly) match the popular WikiText-2 benchmark (Merity et al., 2017). In experiments, we demonstrate that the KGLM achieves significantly better performance than a strong baseline language model. We additionally compare different language models’ ability to complete sentences requiring factual knowledge, and show that the KGLM outperforms even very large language models in generating facts.



• Linguistic Knowledge and Transferability of Contextual Representations
  Nelson F. Liu, Matt Gardner, Yonatan Belinkov, Matthew E. Peters, and Noah A. Smith.
  In North American Chapter of the Association for Computational Linguistics (NAACL), 2019.
  [bib] [abstract] [slides: pdf, pdf with notes, key] [code]

  Contextual word representations derived from large-scale neural language models are successful across a diverse set of NLP tasks, suggesting that they encode useful and transferable features of language. To shed light on the linguistic knowledge they capture, we study the representations produced by several recent pretrained contextualizers (variants of ELMo, the OpenAI transformer LM, and BERT) with a suite of seventeen diverse probing tasks. We find that linear models trained on top of frozen contextual representations are competitive with state-of-the-art task-specific models in many cases, but fail on tasks requiring fine-grained linguistic knowledge (e.g., conjunct identification). To investigate the transferability of contextual word representations, we quantify differences in the transferability of individual layers within contextualizers, especially between RNNs and transformers. For instance, higher layers of RNNs are more task-specific, while transformer layers do not exhibit the same monotonic trend. In addition, to better understand what makes contextual word representations transferable, we compare language model pretraining with eleven supervised pretraining tasks. For any given task, pretraining on a closely related task yields better performance than language model pretraining (which is better on average) when the pretraining dataset is fixed. However, language model pretraining on more data gives the best results.



• Inoculation by Fine-Tuning: A Method for Analyzing Challenge Datasets
  Nelson F. Liu, Roy Schwartz, and Noah A. Smith.
  In North American Chapter of the Association for Computational Linguistics (NAACL), 2019.
  [bib] [abstract] [slides: pdf, pdf with notes, key] [code]

  Several datasets have recently been constructed to expose brittleness in models trained on existing benchmarks. While model performance on these challenge datasets is significantly lower compared to the original benchmark, it is unclear what particular weaknesses they reveal. For example, a challenge dataset may be difficult because it targets phenomena that current models cannot capture, or because it simply exploits blind spots in a model's specific training set. We introduce inoculation by fine-tuning, a new analysis method for studying challenge datasets by exposing models (the metaphorical patient) to a small amount of data from the challenge dataset (a metaphorical pathogen) and assessing how well they can adapt. We apply our method to analyze the NLI "stress tests" (Naik et al., 2018) and the Adversarial SQuAD dataset (Jia and Liang, 2017). We show that after slight exposure, some of these datasets are no longer challenging, while others remain difficult. Our results indicate that failures on challenge datasets may lead to very different conclusions about models, training datasets, and the challenge datasets themselves.



• LSTMs Exploit Linguistic Attributes of Data
  Nelson F. Liu, Omer Levy, Roy Schwartz, Chenhao Tan, and Noah A. Smith.
  In ACL Workshop on Representation Learning for NLP (RepL4NLP), 2018. (Best Paper Award).
  [bib] [abstract] [(short) slides] [poster] [code]

  While recurrent neural networks have found success in a variety of natural language processing applications, they are general models of sequential data. We investigate how the properties of natural language data affect an LSTM's ability to learn a nonlinguistic task: recalling elements from its input. We find that models trained on natural language data are able to recall tokens from much longer sequences than models trained on non-language sequential data. Furthermore, we show that the LSTM learns to solve the memorization task by explicitly using a subset of its neurons to count timesteps in the input. We hypothesize that the patterns and structure in natural language data enable LSTMs to learn by providing approximate ways of reducing loss, but understanding the effect of different training data on the learnability of LSTMs remains an open question.



• AllenNLP: A Deep Semantic Natural Language Processing Platform
  Matt Gardner, Joel Grus, Mark Neumann, Oyvind Tafjord, Pradeep Dasigi, Nelson F. Liu, Matthew Peters, Michael Schmitz, and Luke Zettlemoyer.
  In ACL Workshop for Natural Language Processing Open Source Software (NLP-OSS), 2018.
  [bib] [abstract] [code]

  This paper describes AllenNLP, a platform for research on deep learning methods in natural language understanding. AllenNLP is designed to support researchers who want to build novel language understanding models quickly and easily. It is built on top of PyTorch, allowing for dynamic computation graphs, and provides (1) a flexible data API that handles intelligent batching and padding, (2) high-level abstractions for common operations in working with text, and (3) a modular and extensible experiment framework that makes doing good science easy. It also includes reference implementations of high quality approaches for both core semantic problems (e.g. semantic role labeling (Palmer et al., 2005)) and language understanding applications (e.g. machine comprehension (Rajpurkar et al., 2016)). AllenNLP is an ongoing open-source effort maintained by engineers and researchers at the Allen Institute for Artificial Intelligence.



• Discovering Phonesthemes with Sparse Regularization
  Nelson F. Liu, Gina-Anne Levow, and Noah A. Smith.
  In NAACL Workshop on Subword and Character Level Models in NLP (SCLeM), 2018.
  [bib] [abstract] [poster] [code]

  We introduce a simple method for extracting non-arbitrary form-meaning representations from a collection of semantic vectors. We treat the problem as one of feature selection for a model trained to predict word vectors from subword features. We apply this model to the problem of automatically discovering phonesthemes, which are submorphemic sound clusters that appear in words with similar meaning. Many of our model-predicted phonesthemes overlap with those proposed in the linguistics literature, and we validate our approach with human judgments.



• Crowdsourcing Multiple Choice Science Questions
  Johannes Welbl, Nelson F. Liu, and Matt Gardner.
  In EMNLP Workshop on Noisy User-generated Text, 2017.
  [bib] [abstract] [data] [poster]

  We present a novel method for obtaining high-quality, domain-targeted multiple choice questions from crowd workers. Generating these questions can be difficult without trading away originality, relevance or diversity in the answer options. Our method addresses these problems by leveraging a large corpus of domain-specific text and a small set of existing questions. It produces model suggestions for document selection and answer distractor choice which aid the human question generation process. With this method we have assembled SciQ, a dataset of 13.7K multiple choice science exam questions (Dataset available at data.allenai.org/sciq/). We demonstrate that the method produces in-domain questions by providing an analysis of this new dataset and by showing that humans cannot distinguish the crowdsourced questions from original questions. When using SciQ as additional training data to existing questions, we observe accuracy improvements on real science exams.