No. 227 · Evidence · Fallacy
Base Rate Fallacy
A vivid detail means little until you know how common the outcome is in general.
In the Almanac
“The Sally Clark case”, 1999 · “Kraft Heinz”, 2015
Both stories in full, three ways to use it and where it misleads.
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Questions
What is Base Rate Fallacy?
The base rate fallacy happens when a striking detail makes us forget the background odds. Without knowing how common an outcome is overall, even accurate evidence, like a positive test or a strong pitch, can lead to the wrong conclusion.
How do you avoid Base Rate Fallacy?
Before judging, ask the base rate question: how often does this happen in general? Start from those odds, then adjust for the specifics. Move away from them only when the evidence is genuinely strong.
When does Base Rate Fallacy matter?
Base Rate Fallacy is one of the 68 mental models for evidence, the situations where you ask: Is this really true? Within that, it belongs to “Reading the numbers”, alongside Bayes’ Theorem, Central Limit Theorem and Conditional Probability.
What is an example of Base Rate Fallacy?
Two stories show it at work: The Sally Clark case (1999) and Kraft Heinz (2015). Both are told in full in The Mental Models Daily Almanac, which has two stories for each of its 365 models.
Which mental models are related to Base Rate Fallacy?
Bayes’ Theorem, Availability Bias and Narrative Instinct. Base Rate Fallacy itself comes from fallacy.