Quality in Ranking

A good ranking is what all searchers wants back from a query.

A widely used measure for prediction are Precision and Recall.

In a few words:

Precision measures the hit rate over the times you shoot. More you shoot, more easily you can hit, but you have a low Precision
Recall measures the hit rate over the hittable. You can reach the maximal Recall hitting all the targets, no problem how many tries you did.

It is quite obvious that shooting a lot you can have a good Recall and a bad Precision, on the opposite side, shooting only a few good slugs you will have a good Precision, but a small Recall.

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