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Base Rate Neglect

Also Known As: Base rate fallacy Base rate bias
Cognitive Bias ID: base_rate_neglect

Definition

The tendency to ignore general prevalence information (base rates) when evaluating the probability of a specific event, especially when vivid individuating information is available. People focus on specific case details while ignoring how common or rare a condition is in the general population. This leads to dramatic probability estimation errors.

Examples

A medical test for a rare disease (affecting 1 in 10,000 people) has a 5% false positive rate. When a patient tests positive, both the patient and doctor may assume there is a 95% chance they have the disease, when in reality the base rate makes it far more likely to be a false positive.

A security algorithm flags a traveler as a potential threat based on a profile match. The agency treats the flag as near-certain evidence of guilt, but fails to account for the fact that genuine threats are extraordinarily rare among millions of travelers, meaning most flags are false positives.

A startup investor hears a passionate pitch and thinks the company has a 70% chance of success because the founder seems brilliant and the product is innovative. She ignores the well-documented base rate that roughly 90% of startups fail within ten years, regardless of founder quality.

Verification Steps
Verification Steps
Binary yes/no questions that an AI must answer to detect a reasoning pattern in a text.
Each of the 452 aspects has verification steps — simple yes/no questions designed to systematically detect whether a pattern appears in a text. For ad hominem: "Does the argument attack a person rather than their claim?" For false dichotomy: "Are only two options presented when more exist?" This ensures consistent, reproducible analysis.

Binary (yes/no) questions an LLM must answer to identify this aspect:

  1. 1

    Are background statistics being ignored in favor of vivid case details?

    Type: binary
  2. 2

    Is the overall prevalence of the condition or event being considered?

    Type: binary
  3. 3

    Would the probability estimate change significantly if base rates were included?

    Type: binary
Deep Dive
The expandable detail section on each aspect page with examples, psychology, and counter-strategies.
The Deep Dive section provides in-depth information about each aspect: a real-world example showing the pattern in action, an explanation of why it works psychologically, practical advice on how to counter it, alternative names, and links to related aspects.

Hierarchical Context