The Video Assistant · Lesson 11 — The Trigger Finger ← Course

The trigger finger — precision and recall.

Two officials, the same accuracy, opposite habits. One number can't tell them apart. Two sharper ones can.

The confusion matrix gives you two questions far sharper than accuracy. Precision asks: when the VAR chalks a goal off, how often is it truly a bad goal? — of all its NO GOAL calls, the share that were right. Recall asks: of all the goals that should not have counted, how many did it actually catch?

They are not the same, and a model can be strong at one and weak at the other. Here are two officials judging the same twenty appeals. Both score 70% accuracy — identical. Yet one is trigger-happy and one is cautious, and precision and recall pull them apart at a glance.

High precision means few good goals chalked off — you can trust a NO GOAL call. High recall means few bad goals slip through — invalid goals rarely stand. As you will see, each VAR has bought one by giving up the other.

Switch between the two officials and watch accuracy hold at 70% while precision and recall trade places.

truly NO GOAL
truly a GOAL
said NO GOAL
correct call
0
wrong call
0
said GOAL
missed error
0
correct goal
0
Accuracy same for both70%
Precision of its NO GOAL calls, how many were right0%
Recall of real bad goals, how many it caught0%

Precision and recall see what accuracy hides.

Same 70% accuracy, opposite officials. Precision — of the NO GOAL calls, how many were right — rewards not crying wolf. Recall — of the goals that should not have counted, how many were caught — rewards letting none slip. The trigger-happy VAR wins recall and loses precision; the cautious one does the exact reverse.

So report both, never accuracy alone. A single number can look healthy while the model is quietly making the one mistake that matters to you. Which of the two you should favour — precision or recall — is the last question of this lesson.

Next: the balance →
▸ Go deeper — the formulas, in plain terms optional

Precision and recall as fractions

precision = TP / (TP + FP) — correct calls out of every NO GOAL given. It reads only the top row of the matrix: of the times the VAR chalked a goal off, how many were right. recall = TP / (TP + FN) — correct calls out of every real bad goal. It reads only the left column: of the goals that should not have counted, how many were caught.

Each ignores one mistake

Precision never looks at missed errors (false negatives); recall never looks at wrong calls (false positives). That is precisely why you need both — each is blind to the error the other watches.

Other names

Recall is also called sensitivity or the true positive rate. And all of this hangs on which class you call “positive”: here it is NO GOAL, so precision and recall are about catching the goals that should not count, not the good ones.