Retrieving a Set, Not Independent Passages: Set-Level Compatibility Learning for Efficient Set Exploration

2026-07-06

AI Summary

Traditional retrievers score passages independently, failing to model the interdependencies among evidence pieces in multi-hop question answering, which limits retrieval effectiveness. This work proposes a set-oriented compatibility learning framework that reframes multi-hop retrieval as a holistic compatibility scoring task between the query and a complete evidence set, thereby moving beyond the paradigm of independent passage scoring. Two set-based scorers are introduced: ParaSet, a lightweight late-interaction model leveraging dual-encoder embeddings and self-attention, and SetCE, a cross-encoder reranker trained with the same set-level objective. Experiments demonstrate that both models substantially improve retrieval and downstream QA performance across multiple multi-hop benchmarks. Moreover, their results are complementary, and their fusion outperforms strategies that rely solely on expanding recall with a single retriever.

Abstract

Multi-hop question answering and retrieval-augmented reasoning require selecting evidence passages that are jointly useful for answering a query. However, most retrievers still score passages independently or make locally supervised sequential decisions, which can fail when evidence usefulness depends on compatibility among passages. LLM-based set selection can model such interactions, but its computational cost limits practical use. We address this gap by formulating multi-hop retrieval as query-set compatibility scoring and propose a set-level retrieval framework. Our training objective teaches retrievers to rank complete and compatible evidence sets above incomplete, noisy alternatives, making set scoring more robust to variable-length and partially noisy contexts. We instantiate the framework with two complementary set scorers: ParaSet, a lightweight late-interaction scorer that applies self-attention over precomputed bi-encoder embeddings for fast candidate-set exploration, and SetCE, a cross-encoder-based reranker trained with the same set-level objective. Experiments on various multi-hop QA benchmarks show that set-level compatibility learning improves retrieval performance and downstream QA task performance. We further show that the proposed set-level retrievers not only outperform document-level retrievers, but also exhibit complementary retrieval characteristics: combining their outputs yields stronger performance than simply retrieving more passages from a single document-level retriever.

Problem

• multi-hop retrieval
• set-level compatibility
• evidence passage selection
• retrieval-augmented reasoning
• joint passage usefulness

Innovation

• set-level retrieval
• compatibility learning
• multi-hop QA
• evidence set scoring
• late-interaction