Understand base rates
A sensitive test can produce more false positives than true positives when the condition is rare.
A positive test is not a diagnosis
Experiment with a hypothetical screening test. Adjust how common the condition is, how often the test detects it, and how often it correctly rejects people without it. All counts below are expected counts in 10,000 screenings—not actual patients.
Positive predictive value—
Expected true positives—
Expected false positives—
| Result | Condition present | Condition absent |
|---|---|---|
| Test positive | — | — |
| Test negative | — | — |
This is a mathematical probability model, not medical guidance. Sensitivity and specificity can vary by context; real screening programs also face selection, measurement, and verification bias.