CODEXCard 8 of 8 · Core Cognitive Biases
Base rate neglect
The story drowns out the statistics.
Base rate neglect is the error of judging a case by how well it matches a description while ignoring how common each possibility is to begin with. The base rate is the background frequency, such as how many people in a population have a condition or work in a field. When a vivid story appears, that frequency tends to vanish from the calculation.
Kahneman and Tversky studied this in the 1970s as part of their work on representativeness. The fast mind asks how much a case resembles a stereotype, which is quick and feels insightful. It does not ask how many cases of each kind exist, which requires numbers and effort. A colourful profile drowns out a dull statistic every time.
The opposite mistake is also possible. Waving away real, specific evidence because the base rate is low ignores useful information. The sound approach is a matter of order: begin with the base rate as your starting estimate, then let the particular details move you up or down. Most people skip the first step entirely.
This is close kin to the availability heuristic, since both swap statistics for stories. It also underlies probabilistic thinking, where the question is always what the full range of outcomes looks like before the vivid one grabs your attention.
Example
A friend reads about a rare illness, notices that her fatigue and headaches match the symptom list, and becomes convinced she has it. The description fits, but the condition affects very few people, while tiredness and headaches are extremely common. Starting from how rare the illness is puts the worry in proportion before she sees a doctor.
Try it this week
When a story convinces you this week, ask how common that outcome is in general before you let the details decide.
From Core Cognitive Biases (Daniel Kahneman · 2011).
