BELLE: A Bi-Level Multi-Agent Reasoning Framework for Multi-Hop Question Answering

Multi-hop question answering (QA) involves finding multiple relevant passages and performing step-by-step reasoning to answer complex questions. Previous works on multi-hop QA employ specific methods from different modeling perspectives based on large language models (LLMs), regardless of question types. In this paper, we first conduct an in-depth analysis of public multi-hop QA benchmarks, categorizing questions into four types and evaluating five types of cutting-edge methods: Chainof-Thought (CoT), Single-step, Iterative-step, Sub-step, and Adaptive-step. We find that different types of multi-hop questions exhibit varying degrees of sensitivity to different types of methods. Thus, we propose a Bi-levEL muLti-agEnt reasoning (BELLE) framework to address multi-hop QA by specifically focusing on the correspondence between question types and methods, with each type of method regarded as an "operator" by prompting LLMs differently. The first level of BELLE includes multiple agents that debate to formulate an executable plan of combined "operators" to address the multi-hop QA task comprehensively. During the debate, in addition to the basic roles of affirmative debater, negative debater, and judge, at the second level, we further leverage fast and slow debaters to monitor whether changes in viewpoints are reasonable. Extensive experiments demonstrate that BELLE significantly outperforms strong baselines in various datasets. Additionally, the model consumption of BELLE is higher cost-effectiveness than that of single models in more complex multihop QA scenarios.