Intelligently Weighting Multiple Reference Models for Direct Preference Optimization of LLMs

Skyler Wu; Aymen Echarghaoui

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arXiv · 2512.10040

Intelligently Weighting Multiple Reference Models for Direct Preference Optimization of LLMs

Skyler Wu·Aymen Echarghaoui

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Abstract

Fine-tuning is integral for aligning large language models (LLMs) with human preferences. Multiple-Reference Preference Optimization (MRPO) builds on Direct Preference Optimization (DPO) by fine-tuning LLMs on preference datasets while regularizing the policy towards a mixture of reference models to leverage their collective desirable properties. However, current methods for setting the reference weights are ad-hoc and statistically unsound, leading to unreliable performance. To address this, we introduce four new weighting strategies: two offline methods that leverage held-out validation signal; one online method that uses a sliding-window estimator to reduce overfitting; and an online method that treats reference weighting as a $K$-armed bandit via Thompson Sampling. Experiments using Qwen2.5-0.5B as the policy model and seven reference models from the Llama, Mistral, Qwen, Yi, and Phi families (0.5B-14B each) show that all 4 of our strategies outperform the current MRPO weighting methods on UltraFeedback and SafeRLHF in preference accuracy. More thought-provokingly, however, we find that single-reference DPO, using any of 6 out of 7 references, consistently outperforms all tested multiple-reference approaches -- calling into question the practical appeal of multiple-reference approaches.

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Skyler Wu, Aymen Echarghaoui. 2025-12-10. Intelligently Weighting Multiple Reference Models for Direct Preference Optimization of LLMs. https://arxiv.org/abs/2512.10040

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