Why a 99% Accurate Medical Test Can Still Mislead You About Your Results

A common probability puzzle asks what it really means to test positive for a rare condition when the test itself is highly accurate. This video walks through the question step by step, comparing intuitive guesses with a careful calculation, and shows how the final figure can differ sharply from what most people expect.

■ The probability puzzle
- The setup: a condition affecting one in a thousand people, a 99% accurate test, and a 1% false positive rate
- Several first impressions of the odds, including answers from a doctor
- A step-by-step breakdown using a large hypothetical population
- Comparing the number of true positives with the number of false positives
- The resulting probability and why it falls below the intuitive answer

■ Key takeaways
- How accuracy and false positive rates interact
- Why a high-accuracy test can still produce a misleading result

Recommended for viewers interested in statistics, medical testing, or everyday reasoning under uncertainty. Watching helps you practice questioning surface-level numbers before acting on them, and gives you a concrete example to reuse when similar figures appear in news or health decisions.