Title: Adaptive Margin RLHF via Preference over Preferences
Primary area: alignment, fairness, safety, privacy, and societal considerations
Keywords: Alignment, Reinforcement Learning from Human Feedback, Large Language Models

Abstract: Margin-based optimization is fundamental to improving generalization and robustness in classification tasks. In the context of reward model learning from preferences within Reinforcement Learning from Human Feedback (RLHF), existing methods typically rely on no margins, fixed margins, or margins that are simplistic functions of preference ratings. However, such formulations often fail to account for the varying strengths of different preferences—i.e., some preferences are associated with larger margins between responses— or they rely on noisy margin information derived from preference ratings. In this work, we argue that modeling the strength of preferences can lead to better generalization and more faithful alignment with human intent. Furthermore, many existing methods that use adaptive margins assume access to accurate preference scores, which can be difficult for humans to provide reliably.
We propose a novel approach that leverages preferences over preferences—that is, annotations indicating which of two preferences reflects a stronger distinction. We use this ordinal signal to infer adaptive margins on a per-datapoint basis. We introduce an extension to Direct Preference Optimization (DPO), named DPO-PoP, that incorporates adaptive margins from preference-over-preference supervision, enabling improved discriminative and generative performance. Empirically, our method outperforms vanilla DPO, DPO with fixed margins, and DPO with ground-truth margins, on the UltraFeedback dataset. 
These results suggest that integrating preference-over-preference information—which requires less precision to be provided accurately—can improve discriminative and generative performance without adding significant complexity. Additionally, we show that there is a tradeoff between discriminative and generative performance: improving test classification accuracy, particularly by correctly labeling weaker preferences at the expense of stronger ones, can lead to a decline in generative quality. To navigate this tradeoff, we propose two sampling strategies to gather preference-over-preference labels: one favoring discriminative performance and one favoring generative performance.

TL;DR: The paper proposes a way to do margin aware alignment using preference over preference supervision, improving both generative and discriminative performance of LLMs