Base Rate Calculator
A test is 99% accurate and comes back positive. Work out what that actually means.
Requires KIT Apps · Runs locally · 14 KBKIT Apps is free
What you can explore
- Set how common the condition is, and how good the test is at finding it.
- Read the probability that a positive result is actually a true positive.
- See the same numbers as a population of 10,000 people, which is where the intuition lands.
- Drop the base rate and watch a very good test become nearly uninformative.
How it works
Bayes' theorem, shown two ways. The probability that a positive result is real is the number of true positives divided by all positives — true positives plus false ones. When the condition is rare, the false positives are drawn from a much larger group than the true positives, so they can outnumber them even when the test is excellent. The natural-frequency view exists because people get this right far more often when it is counted out in people than when it is written as a conditional probability.