Precision vs Recall

Precision asks: of everything I flagged, how much was real? — TP / (TP + FP). For a spam filter: of the mail sent to the spam folder, how much was actually spam? High precision means few false accusations.

Recall asks: of all the real positives out there, how many did I catch? — TP / (TP + FN). For a cancer screen: of all actual tumors, how many were found? High recall means few misses.

Both matter. Almost never at the same time.