Drag two sliders — how common a thing is, and how accurate the test is — and watch most “positive” results turn out to be false alarms. The rarer the thing, the more a single alarm lies to you.
Interactive — the base-rate machinedrag the sliders
Before the test
4%
the base rate
After a positive
29%
chance it's real
Out of 100, 14 come back positive — but only 4 are actually the real thing.
A 90%-accurate test moved you from 4% to 29% — a real update, but a “positive” is still more likely a false alarm than the real thing. The rarer the thing, the more a single alarm lies.
Works for anything that flags rare things: medical screens, spam filters, fraud alerts, lie detectors. “Accurate” is one number standing in for both sides — how often the test is right when it isthe real thing, and when it isn't.
- real & flagged
- false alarm
- real, missed
- correctly clear