Surrogate Signals from Format and Length: Reinforcement Learning for
  Solving Mathematical Problems without Ground Truth Answers
Authors: name: Rihui Xin; name: Han Liu; name: Zecheng Wang; name: Yupeng Zhang; name: Dianbo Sui; name: Xiaolin Hu; name: Bingning Wang
Published: 2025-05-26 02:56:22

Abstract: Large Language Models have achieved remarkable success in natural language
processing tasks, with Reinforcement Learning playing a key role in adapting
them to specific applications. However, obtaining ground truth answers for
training LLMs in mathematical problem-solving is often challenging, costly, and
sometimes unfeasible. This research delves into the utilization of format and
length as surrogate signals to train LLMs for mathematical problem-solving,
bypassing the need for traditional ground truth answers.Our study shows that a
reward function centered on format correctness alone can yield performance
improvements comparable to the standard GRPO algorithm in early phases.
Recognizing the limitations of format-only rewards in the later phases, we
incorporate length-based rewards. The resulting GRPO approach, leveraging
format-length surrogate signals, not only matches but surpasses the performance
of the standard GRPO algorithm relying on ground truth answers in certain
scenarios, achieving 40.0\% accuracy on AIME2024 with a 7B base model. Through
systematic exploration and experimentation, this research not only offers a
practical solution for training LLMs to solve mathematical problems and
reducing the dependence on extensive ground truth data collection, but also
reveals the essence of why our label-free approach succeeds: base model is like
an excellent student who has already mastered mathematical and logical
reasoning skills, but performs poorly on the test paper, it simply needs to
develop good answering habits to achieve outstanding results in exams , in
other words, to unlock the capabilities it already possesses.