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EX-15 · Make the dense leg earn its keep

Difficulty: ⚫ · label difficulty: 4 Stuck? Ask in the clinic thread for EX-15 — one long-running thread per exercise, so the answers accumulate where the next person will look. Submit: post it in Show and Tell titled [solution · EX-15] ..., with your numbers and your interval.

Notebook 04 · open-ended · Seam ② · Skill: understanding why embeddings work at all

The offline LSA encoder is genuinely weaker than BM25 here. Close the gap without adding a neural model: better vocabulary, n-grams, term weighting, dimension routing, or corpus augmentation.

Acceptance criteria

  • Dense-only evidence recall, before and after, sliced by query class
  • An explanation of the mechanism — why your change helped
  • Whether the hybrid improves once the dense leg does