F1 Is a Summary - Evaluation, Honestly

F1 combines precision and recall into one exact rational value for each listed threshold. This lesson keeps the counts and the two source rates visible beside the summary.

highlighted = computed this step

F1 combines two rates

F1 is computed after precision and recall are already known. The formula is 2*precision*recall divided by precision+recall. It is a summary row, not a new count.

F_{1}={2PR\over P+R}

High threshold

At threshold 5, precision is 1 and recall is 1/3. The exact F1 value is 1/2.

F_{1}={2\cdot1\cdot1/3\over1 + 1/3}=1/2

Middle and low thresholds

At threshold 3, precision and recall are both 2/3, so F1 is 2/3. At threshold 1, precision is 3/5 and recall is 1, so F1 is 3/4.

t=3\Rightarrow F_{1}=2/3,\quad t=1\Rightarrow F_{1}=3/4

Keep the counts visible

The F1 column is useful, but it is still a summary of the listed precision and recall. The counts stay visible because F1 alone does not show which errors moved, and it does not prove future performance, training behavior, or generalization.

\text{F}_{1}\text{ summary}\ne\text{full evaluation story}