Lesson 2: Hybrid search: merging keyword and vector rankings
31 min read·13 Oct 2026
Module 6 · Lesson 2 · About 50 minutes · Needs: Lesson 1 of this module (BM25 and dense retrieval) and Module 4 Lesson 4 (recall@k, MRR, the paired bootstrap)
The situation
Lesson 1 left Hollin with two retrievers that fail in different places. Grace's agents type both kinds of question: part and case numbers, and plain sentences. Her question: run both and merge them?
The running example is q23, an agent's question: "is blade wear covered for the X200". BM25 puts the current warranty terms first; dense search puts them sixth. This lesson merges the two lists four ways, measures each on the 32 labelled questions, and decides. The Hollin data is simulated.