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What is a Base Rate Fallacy?

 , February 10, 2016 updated on February 04, 2018
The base rate fallacy is a tendency to focus on specific information over general probabilities. For example:
1 in 1000 students cheat on an exam
A cheating detection system catches cheaters with a 5% false positive rate
All 1000 students are tested by the system
The cheating detection system catches Sara
What is the chance that Sara is innocent?
Many people who answer the question focus on the 5% false positive rate and exclude the general statistic that 999 out of 1000 students are innocent. This results in the incorrect answer that there is a 5% chance that Sara is innocent. This incorrect answer neglects the fact that by testing 1000 students, the machine will catch 50 students on average and 49 of these students will be innocent. Therefore, there is approximately a 98% chance that Sara is innocent.
Overview: Base Rate Fallacy
DefinitionFocusing on a specific probability while ignoring a more general probability.
Related ConceptsProsecutors Fallacy

Cognitive Biases

This is the complete list of articles we have written about cognitive biases.
Ambiguity Effect
Backfire Effect
Base Rate
Circular Reasoning
Cognitive Bias
Cognitive Dissonance
Complexity Bias
Crab Mentality
Creeping Normality
Curse Of Knowledge
Decoy Effect
False Analogy
False Hope
Fear Of Youth
Golden Hammer
Halo Effect
Hindsight Bias
Negativity Bias
Optimism Bias
Peak-End Rule
Positive Bias
Sour Grapes
Survivorship Bias
Us vs Them
Victim Mentality
Wishful Thinking
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Cognitive Biases

A list of common cognitive biases explained.

Curse Of Knowledge

Why experts have trouble communicating.

Optimism Bias

An overview of optimism bias, including its surprising benefits.

Decoy Effect

A cognitive bias that is well known in marketing circles.

Biases vs Heuristics

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Information Cascade

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Functional Fixedness

A definition of functional fixedness with examples.

Boil The Frog

A definition of boil the frog, with examples.

Anecdotal Evidence

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Thought Processes

A list of thinking approaches and types.


A few logic terms explained.

Abstract Ideas

A few dangers of being too abstract.

Objective vs Subjective

The difference between objective and subjective.

Intellectual Diversity

A definition of intellectual diversity with examples.

Creative Value

The definition of creative value with examples.

Benefit Of Doubt

The definition of benefit of doubt with examples.


The definition of pessimism with examples.
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