 Recall (information retrieval)

Recall measures how much of the relevant material an information retrieval system finds. For a relevant-document set $R$ and returned-document set $S$, it is $|R\cap S|/|R|$, provided $R$ is nonempty.^[ref]

Suppose $R=\{2,4,6\}$ and $S=\{1,2,3,4,5\}$. Two of three relevant documents were found, so recall is $2/3$. Adding document six raises it to one, while precision becomes $3/6$. Returning the whole collection guarantees complete coverage of the assessed relevant set, but can burden the user with many irrelevant results. Estimation requires relevance judgments beyond the returned documents; empty relevant sets need an explicit evaluation convention.