Tyler A. Chang


2024

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Correlations between Multilingual Language Model Geometry and Crosslingual Transfer Performance
Cheril Shah | Yashashree Chandak | Atharv Mahesh Mane | Benjamin Bergen | Tyler A. Chang
Proceedings of the 2024 Joint International Conference on Computational Linguistics, Language Resources and Evaluation (LREC-COLING 2024)

A common approach to interpreting multilingual language models is to evaluate their internal representations. For example, studies have found that languages occupy distinct subspaces in the models’ representation spaces, and geometric distances between languages often reflect linguistic properties such as language families and typological features. In our work, we investigate whether geometric distances between language representations correlate with zero-shot crosslingual transfer performance for POS-tagging and NER in three multilingual language models. We consider four distance metrics, including new metrics that identify a basis for a multilingual representation space that sorts axes based on their language-separability. We find that each distance metric either only moderately correlates or does not correlate with crosslingual transfer performance, and metrics do not generalize well across models, layers, and tasks. Although pairwise language separability is a reasonable predictor of crosslingual transfer, representational geometry overall is an inconsistent predictor for the crosslingual performance of multilingual language models.

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Detecting Hallucination and Coverage Errors in Retrieval Augmented Generation for Controversial Topics
Tyler A. Chang | Katrin Tomanek | Jessica Hoffmann | Nithum Thain | Erin MacMurray van Liemt | Kathleen Meier-Hellstern | Lucas Dixon
Proceedings of the 2024 Joint International Conference on Computational Linguistics, Language Resources and Evaluation (LREC-COLING 2024)

We explore a strategy to handle controversial topics in LLM-based chatbots based on Wikipedia’s Neutral Point of View (NPOV) principle: acknowledge the absence of a single true answer and surface multiple perspectives. We frame this as retrieval augmented generation, where perspectives are retrieved from a knowledge base and the LLM is tasked with generating a fluent and faithful response from the given perspectives. As a starting point, we use a deterministic retrieval system and then focus on common LLM failure modes that arise during this approach to text generation, namely hallucination and coverage errors. We propose and evaluate three methods to detect such errors based on (1) word-overlap, (2) salience, and (3) LLM-based classifiers. Our results demonstrate that LLM-based classifiers, even when trained only on synthetic errors, achieve high error detection performance, with ROC AUC scores of 95.3% for hallucination and 90.5% for coverage error detection on unambiguous error cases. We show that when no training data is available, our other methods still yield good results on hallucination (84.0%) and coverage error (85.2%) detection.

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A Bit of a Problem: Measurement Disparities in Dataset Sizes across Languages
Catherine Arnett | Tyler A. Chang | Benjamin Bergen
Proceedings of the 3rd Annual Meeting of the Special Interest Group on Under-resourced Languages @ LREC-COLING 2024

How should text dataset sizes be compared across languages? Even for content-matched (parallel) corpora, UTF-8 encoded text can require a dramatically different number of bytes for different languages. In our work, we define the byte premium between two languages as the ratio of bytes used to encode content-matched text in those languages. We compute byte premiums for 1155 languages, and we use linear regressions to estimate byte premiums for other languages. We release a tool to obtain byte premiums for any two languages, enabling comparisons of dataset sizes across languages for more equitable multilingual model development and data practices.

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Language Model Behavior: A Comprehensive Survey
Tyler A. Chang | Benjamin K. Bergen
Computational Linguistics, Volume 50, Issue 1 - March 2024

Transformer language models have received widespread public attention, yet their generated text is often surprising even to NLP researchers. In this survey, we discuss over 250 recent studies of English language model behavior before task-specific fine-tuning. Language models possess basic capabilities in syntax, semantics, pragmatics, world knowledge, and reasoning, but these capabilities are sensitive to specific inputs and surface features. Despite dramatic increases in generated text quality as models scale to hundreds of billions of parameters, the models are still prone to unfactual responses, commonsense errors, memorized text, and social biases. Many of these weaknesses can be framed as over-generalizations or under-generalizations of learned patterns in text. We synthesize recent results to highlight what is currently known about large language model capabilities, thus providing a resource for applied work and for research in adjacent fields that use language models.

2022

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Word Acquisition in Neural Language Models
Tyler A. Chang | Benjamin K. Bergen
Transactions of the Association for Computational Linguistics, Volume 10

We investigate how neural language models acquire individual words during training, extracting learning curves and ages of acquisition for over 600 words on the MacArthur-Bates Communicative Development Inventory (Fenson et al., 2007). Drawing on studies of word acquisition in children, we evaluate multiple predictors for words’ ages of acquisition in LSTMs, BERT, and GPT-2. We find that the effects of concreteness, word length, and lexical class are pointedly different in children and language models, reinforcing the importance of interaction and sensorimotor experience in child language acquisition. Language models rely far more on word frequency than children, but, like children, they exhibit slower learning of words in longer utterances. Interestingly, models follow consistent patterns during training for both unidirectional and bidirectional models, and for both LSTM and Transformer architectures. Models predict based on unigram token frequencies early in training, before transitioning loosely to bigram probabilities, eventually converging on more nuanced predictions. These results shed light on the role of distributional learning mechanisms in children, while also providing insights for more human-like language acquisition in language models.

2020

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Encodings of Source Syntax: Similarities in NMT Representations Across Target Languages
Tyler A. Chang | Anna Rafferty
Proceedings of the 5th Workshop on Representation Learning for NLP

We train neural machine translation (NMT) models from English to six target languages, using NMT encoder representations to predict ancestor constituent labels of source language words. We find that NMT encoders learn similar source syntax regardless of NMT target language, relying on explicit morphosyntactic cues to extract syntactic features from source sentences. Furthermore, the NMT encoders outperform RNNs trained directly on several of the constituent label prediction tasks, suggesting that NMT encoder representations can be used effectively for natural language tasks involving syntax. However, both the NMT encoders and the directly-trained RNNs learn substantially different syntactic information from a probabilistic context-free grammar (PCFG) parser. Despite lower overall accuracy scores, the PCFG often performs well on sentences for which the RNN-based models perform poorly, suggesting that RNN architectures are constrained in the types of syntax they can learn.