What do you understand by Precision and Recall?

Formulas for precision and recall

In order to understand the difference between Precision and Recall, We can consider this analogy:

Imagine that, your girlfriend gave you a birthday surprise every year since last 10years. One day, your girlfriend asks you: “Honey, do you remember all birthday surprises from me? Now you need to RECALL all 10 surprising events from your memory.

So, “Recall” is the ratio of a number of events you can correctly recall to a number of all correct events. If you can recall all 10 events correctly, then your recall ratio is 1.0(100%). If you can recall 7 events correctly, your recall ratio is 0.7(70%). However, You might be wrong in some answers.

For example- You answered 15 times, 10 events are correct and 5 events are wrong. This means you can recall all events but its not so PRECISE.

So, “Precision” is the ratio of a number of events you can correctly recall to a number all events you recall(mix of correct and wrong recalls).

From last example(10 real events, 15 answers: 10 correct 5, wrong), you get 100% recall but your precision is only 66.67%(10/15).

1. Number of events you can correctly recall= True Positives(they are correct and you recall them).
2. 2. Number of all correct events= True positives(they are correct and you recall them) + False negatives(they are correct but you don't recall them).
3. 3. Number of all events you recall = True Positives(they are correct and you recall them) + False Positives(they are not correct but you recall them)
4. RECALL = TRUE POSITIVE/(TRUE POSITIVE PLUS FALSE NEGATIVE)
5. PRECISION= TRUE POSITIVE / ( TRUE POSITIVE PLUS FALSE POSITIVE)