Precision at n

Synonyms

P@n

Definition

In an information retrieval system that retrieves a ranked list, the top-n documents are the first n in the ranking. Precision at n is the proportion of the top-n documents that are relevant.

Key Points

If r relevant documents have been retrieved at rank n, then:

$${\rm{\ Precision\ at\ n}} =\, {{r}\over {n}}$$

The value of n can be chosen based on an assumption about how many documents the user will view. In Web search a results page typically contains ten results, so n = 10 is a natural choice. However, not all users will use the scrollbar and look at the full top ten list. In a typical setup the user may only see the first five results before scrolling, suggesting Precision at 5 as a measure of the initial set seen by users. It is the document at rank 1 that gets most user attention, because this is the document that users view first, suggesting the use of Precision at 1 (which is equivalent to Success at 1).

It is possible to calculate precision at a later...