Never ranked first. Still won.

transcript

This article never ranked first. It still beat the one that did.

Keyword search finds exact codes, like RTN-4471. Vector search finds meaning, like paisa wapas.

So you run both. How do you merge them? Just add the scores? No. BM25 is unbounded. Cosine sits in a narrow band. One side dominates.

Reciprocal rank fusion ignores scores. It only uses ranks. Each list gives a document 1 over 60 plus its rank. Add them up. Highest total wins. Why 60? The 2009 paper picked it in a pilot.

Article A: first in keywords, tenth in vectors. Article B: third in both. B wins. Article C: first in vectors, missing from keywords. About half.

The 60 flattens the gap between rank 1 and rank 10. A is first once. B is never first. B still wins. So agreement beats one strong vote.

But C might be the Hinglish answer only vectors could find. So pass the reranker a generous list. Not RRF's top 3.

Full breakdown in the lecture.