This GeoGebra worksheet can be used to explore the following problem, which is a classic application of Bayes' theorem: If a person tests positive for a disease, what is the probability that he or she is actually infected?

The user can enter the following quantities by using the input boxes or the sliders:
  • Incidence is the percentage of the general population that is infected.
  • False positive rate is the probability that a person who is not infected will test positive for the disease.
  • False negative rate is the probability that a person who is infected will test negative for the disease.

The pie chart shows the conditional probability that a person is infected given that he or she tests positive for the disease.

Comments or suggestions are welcome. Enjoy!

Addendum: Dave Richeson had a similar idea.