What is retrieval recall at k and why does it matter?

Retrieval recall at k is the proportion of relevant documents that appear in the top-k retrieved results for a query. For example, recall at 10 of 0.8 means 80 percent of relevant documents are in the top 10. It is the most important retrieval metric because if relevant documents are not retrieved, no amount of reranking or generation can recover them. Measure recall on a labeled evaluation set with at least several dozen queries. Typical production targets are recall at 10 above 0.9 and recall at 5 above 0.8.