The Base-Rate Fallacy: Even with a 99% accurate test, if a condition affects only 1 in 1,000 people (0.1%), testing positive still means you have only an ~8.3% true positive probability because healthy false alarms drastically outnumber true diseased cases.
Base Rate Prevalence
0.1%
Test Sensitivity (True +)
99.0%
False Positive Rate
1.0%
True Positives
1
False Positives
10
Posterior Probability P(Condition | Positive)
9.09%