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Probability: Fallacies, Myths and Puzzles: Main Page

Did the Doctor make the correct medical diagnosis?


Consider the following problem
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"One in a thousand people has a prevalence for a particular heart disease. There is a test to detect this disease. The test is 100% accurate for people who have the disease and is 95% accurate for those who don't (this means that 5% of people who do not have the disease will be wrongly diagnosed as having it).

If a randomly selected person tests positive what is the probability that the person actually has the disease?"

This question was put to 60 students and staff at Harvard Medical School.

Almost half gave the response 95%.

The 'average' answer was 56%.

In fact, the correct answer is very different and was given by just 11 participants.
Click here for the answer and an explanation.



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Norman Fenton


Return to Main Page Making Sense of Probability: Fallacies, Myths and Puzzles