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arXiv · 2406.10203

A Probability--Quality Trade-off in Aligned Language Models and its Relation to Sampling Adaptors

Naaman Tan·Josef Valvoda·Tianyu Liu·Anej Svete·Yanxia Qin·Kan Min-Yen·Ryan Cotterell
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Abstract

The relationship between the quality of a string, as judged by a human reader, and its probability, $p(\boldsymbol{y})$ under a language model undergirds the development of better language models. For example, many popular algorithms for sampling from a language model have been conceived with the goal of manipulating $p(\boldsymbol{y})$ to place higher probability on strings that humans deem of high quality. In this article, we examine the probability--quality relationship in language models explicitly aligned to human preferences, e.g., through reinforcement learning through human feedback. We show that, when sampling corpora from an aligned language model, there exists a trade-off between the strings' average reward and average log-likelihood under the prior language model, i.e., the same model before alignment with human preferences. We provide a formal treatment of this phenomenon and demonstrate how a choice of sampling adaptor allows for a selection of how much likelihood we exchange for the reward.

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Naaman Tan, Josef Valvoda, Tianyu Liu, Anej Svete, Yanxia Qin, Kan Min-Yen, Ryan Cotterell. 2024-10-28. A Probability--Quality Trade-off in Aligned Language Models and its Relation to Sampling Adaptors. https://arxiv.org/abs/2406.10203

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