Reinforcement learning from human feedback (RLHF) and, at its core, reward modeling have become a crucial part of training powerful large language models (LLMs). One commonly overlooked factor in training high-quality reward models (RMs) is the effect of the base model, which is becoming more challenging to choose given the rapidly growing pool of LLMs. In this work, we present a systematic analysis of the effect of base model selection on reward modeling performance. Our results show that the performance can be improved by up to 14% compared to the most common (i.e., default) choice. Moreover, we showcase the strong statistical relation between some existing benchmarks and downstream performances. We also demonstrate that the results from a small set of benchmarks could be combined to boost the model selection (+18% on average in the top 5-10). Lastly, we illustrate the impact of different post-training steps on the final performance and explore using estimated data distributions to reduce performance prediction error.
@article{arxiv.2505.10775,
title = {A Systematic Analysis of Base Model Choice for Reward Modeling},
author = {Kian Ahrabian and Pegah Jandaghi and Negar Mokhberian and Sai Praneeth Karimireddy and Jay Pujara},
journal= {arXiv preprint arXiv:2505.10775},
year = {2025}
}