Visualizing PerformanceBuilding ROC Curves

Each threshold gives one pair of rates, so it gives one point (FPR, TPR) on a graph. Drag the threshold on the score line as before, and watch its point move on the graph below. The thresholds you pass leave a trail. Complete the three missions.

That trail is the ROC curve. It always starts at (0, 0), where nobody tests positive, and ends at (1, 1), where everybody does.

As the threshold slides down past one more person's score, that person starts testing positive. If the person is healthy, the point moves

One more false alarm raises the FPR by 1 ÷ 190, so the point steps right; one more patient caught raises the TPR by 1 ÷ 10, a step up. That is why an ROC curve from real data looks like a staircase.