Hidden Structures in Chaos
Investigating statistical laws that govern seemingly random or chaotic datasets.
The Prosecutor's Fallacy (Base-Rate Neglect)
The Prosecutor's Fallacy, or base-rate neglect, is the mistake of focusing on how strong or unusual one piece of evidence seems while ignoring how common the underlying situation is to begin with. People often confuse "this evidence would be unusual by chance" with "therefore this explanation must be very likely." See how this plays out in two scenarios: a cancer screening test, and a courtroom DNA match.
This is one of the most common reasoning errors in medicine and the courtroom: people hear "the test is 90% accurate" and assume a positive result means a 90% chance of being right. It usually doesn't. What matters just as much as the test's accuracy is how rare the thing it's testing for was to begin with, how common the condition already is in the group being tested. Adjust the sliders below and watch the grid of 10,000 people change color.
Further reading: Base Rate Fallacy on Wikipedia · Prosecutor's Fallacy on Wikipedia.