04 r-precision

-precision is an evaluation metric for ranking models, and is closely related to precision@k. Let be the number of relevant documents for a given query. Then the -precision is the fraction of the first ranked documents that are relevant.

Compared to precision@k, r-precision has the advantage of being invariant to the number of relevant documents. However, for queries with many relevant documents, it can obscure the fact that the most important ones do not rank highly.

-precision can be useful in a research context and as a sanity check for production systems. However, its limitations make it less useful as a key target for production ranking models.