Known-Item Search Success

Known-item search is the case where the user is looking for one specific document they already know exists — a particular paper, book, web page, or record — rather than exploring a topic. Evaluation is correspondingly specialized: with exactly one correct answer per query, the question is simply how high the system ranks that single target. The natural measures are reciprocal rank (and its mean, MRR), success-at-k, and Cooper's expected search length, which counts how many wrong documents the user must wade through before reaching the right one. These metrics, averaged over many known-item topics, give a clean, interpretable picture of how well a system supports re-finding a specific document.