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Rewarding Progress: Scaling Automated Process Verifiers for LLM Reasoning

Paper · 2024 · Amrith Setlur, Chirag Nagpal, Adam Fisch et al.

Summary

Process reward models (PRMs) score every step of a reasoning trace instead of only the final answer. Step labels written by humans are expensive, though, and earlier automatically labelled PRMs gave only marginal gains over outcome reward models (ORMs). This work asks what a step-level reward should actually measure. The authors' answer is that a step should earn credit for progress: how much it raises or lowers the chance of eventually reaching a correct answer. In RL terms this is the step's advantage, and it is computed under a separate "prover" policy rather than the base policy itself. Their theory says useful provers must be able to separate good steps from bad ones produced by the base model while staying broadly aligned with it. Provers that are too strong or too weak fail this test, and even a prover weaker than the base model can improve it. The authors train process advantage verifiers (PAVs) on rollout data collected automatically and test them with Gemma2 models on math reasoning. PAVs make both test-time search and online RL clearly more accurate and more efficient than ORMs, which makes automated dense supervision practical.

Key points

  • Progress as reward: A step's process reward is the change in the prover's success probability before and after the step. Using the base policy's own advantages adds nothing, because it produces the same policy-gradient updates as outcome-only reward.
  • Prover choice: Best-of-K versions of the base policy with a moderate K > 1 work well as provers. A prover that succeeds from almost any step gives every step roughly the same score and cannot tell them apart.
  • Test-time search: Beam search guided by PAVs is more than 8% more accurate than ORM re-ranking and 1.5–5× more compute-efficient.
  • Online RL: Using PAVs as dense rewards gives 5–6× better sample efficiency and more than 6% higher accuracy than ORM-only rewards. It also improves Pass@N and solves hard problems that the SFT policy cannot solve even with N = 256 samples.
  • Setup: Experiments use Gemma2 2B, 9B and 27B. Training data comes from "seed" traces sampled from the prover plus partial rollouts from their prefixes, which give Q-value estimates.

Our summary of the paper, in our own words — not its text. The work belongs to its authors; read it at the links above.

 Institutions

 Fields, topics and concepts

Cite

Cite as Amrith Setlur, Chirag Nagpal, Adam Fisch, Xinyang Geng, Jacob Eisenstein, Rishabh Agarwal, Alekh Agarwal, Jonathan Berant and Aviral Kumar. 2024. Rewarding Progress: Scaling Automated Process Verifiers for LLM Reasoning. arXiv:2410.08146.

The paper itself, not this page. BibTeX from the library's record of the work. DOI 10.48550/arxiv.2410.08146 · arXiv 2410.08146

@misc{setlur2024rewarding,
  title = {{Rewarding Progress: Scaling Automated Process Verifiers for LLM Reasoning}},
  author = {Amrith Setlur, Chirag Nagpal, Adam Fisch, Xinyang Geng, Jacob Eisenstein, Rishabh Agarwal, Alekh Agarwal, Jonathan Berant and Aviral Kumar},
  year = {2024},
  doi = {10.48550/arxiv.2410.08146},
  eprint = {2410.08146},
  archivePrefix = {arXiv},
  url = {https://doi.org/10.48550/arxiv.2410.08146}
}

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