Probability · Bayes with a census
A thousand people, one test
Prior, sensitivity, false positives — counted as people, then as a posterior. The base-rate trap, made visible.
A thousand people walk through the clinic. The posterior is not the sensitivity. It is true positives divided by everyone who tested positive — including the healthy majority who still trigger the test.
The thousand
Sick 10Healthy 990
Among the sick
True + 9False − 1
Among the healthy
False + 50True − 941
P(D | +)
15.4%
9 true positives / 59 positive tests
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A thousand people, one test
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