Evaluating Search — How Do You Know Your Results Are Good?
You changed the ranking function and the results look different — but are they better? Search quality feels subjective, yet the entire field rests on measuring it rigorously, because you cannot improve what you cannot measure. Evaluating search means quantifying relevance with metrics that respect what users actually care about: not just whether the right documents appear, but whether they appear near the top. This post is about measuring relevance honestly, offline and online.
You changed the ranking and results look different — but are they better? Precision and recall ignore order (fatal for a ranked list), so rank-aware metrics (precision@k, MRR, NDCG) rule. Offline test collections for fast iteration, online A/B tests as ground truth — the same metrics that evaluate RAG retrieval.