Suppose your doctor tests you for a terrible disease, and the results are positive! How worried should you be? Bayes' theorem, can be used to answer this question.

Suppose the total population is people. Let D = Diseased and H = Healthy.

The incidence of the disease in the population is
%

The disease testing accuracy is %

- p(D | positive) means the probability of having the disease given that you have tested positive. This is the notation for representing a conditional probability.
- p(positive | D) = p(negative | H) = test accuracy, by definition.
- p(positive) is the total number of people that test positive () divided by the total population () =