Precision and Recall

Synonyms

False negative rate; Positive predictive value; Sensitivity

Definition

Recall measures ?> the ability of a search engine or retrieval system to locate relevant material in its index. Precision measures its ability to not rank nonrelevant material. With everything above rank cut-off n considered “retrieved” and everything below considered “not retrieved,” precision and recall can be stated mathematically as:

$$ \begin{array}{l}\mathrm{precision}=\frac{\left|\mathrm{retrieved} \& \mathrm{relevant} \mathrm{at} \mathrm{rank} n\right|}{\left|\mathrm{retrieved} \mathrm{at} \mathrm{rank} n\right|}\\[8pt] {}\mathrm{recall}=\frac{\left|\mathrm{retrieved} \& \mathrm{relevant} \mathrm{at} \mathrm{rank} n\right|}{\left|\mathrm{relevant}\right|}\end{array} $$

Key Points

Precision and recall are the traditional metrics for retrieval system performance evaluation, and nearly all other performance measures can be seen as either precision-based, recall-based, or a combination of the two.

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