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Do The Math

A blood test indicates the presence of a particular disease 95% of the time when the disease is actually present. The same test indicates the presence of the disease 0.5% of the time when the disease is not present. One percent of the population actually has the disease. Calculate the probability that a person has the disease given that the test indicates the presence of the disease.

  1. 0.324
  2. 0.657
  3. 0.945
  4. 0.950
  5. 0.995

Solution: B
Let
Y = positive test result
D = disease is present (and ~D = not D)
Using Baye’s theorem: