Andy Evans
Research & Writing·6 min read

Base Rates

Why starting with the outside view — population-level frequencies — is the foundation of good probabilistic judgement.

The Linda problem

Consider this description, drawn from Daniel Kahneman’s Thinking, Fast and Slow:

Linda is 31 years old, single, outspoken, and very bright. She majored in philosophy. As a student, she was deeply concerned with issues of discrimination and social justice, and also participated in anti-nuclear demonstrations.

Now answer: which is more likely?

  1. a.Linda is a bank teller.
  2. b.Linda is a bank teller and is active in the feminist movement.

In studies, 85–90% of respondents choose (b). But (b) cannot be more probable than (a) — it is a subset of (a). Being a bank teller and a feminist is one specific way of being a bank teller. The probability of the conjunction can never exceed the probability of either component alone.

This is the conjunction fallacy — a vivid instance of base rate neglect. The description of Linda is so evocative, so representative of a feminist activist, that people ignore the simple probabilistic structure and chase the narrative. The base rate — what is the frequency of bank tellers vs. feminist bank tellers in the population — is overwhelmed by the story.

Kahneman’s point is fundamental: when the inside view (the narrative, the case-specific detail) is vivid, humans systematically neglect the outside view (the frequency, the base rate, the statistical structure). This is not a failure of intelligence. It is a failure of System 1 — the fast, intuitive, pattern-matching mode of thought that dominates most of our reasoning.

Test your intuitions

The Linda problem is the theory. Now try it on yourself. The following six questions — inspired by Hans Rosling’s work in Factfulnessand drawn from global development data and investment research — are chosen because most people get them wrong. With three options per question, random guessing scores 33%. In Rosling’s audiences of Nobel laureates, business leaders, and senior politicians, the average score was consistently below that.

Test your intuitions

1 / 6

In the last 30 years, the proportion of the world population living in extreme poverty has…

Base rates in investing

The base rate fallacy is pervasive in investment management. Consider these questions:

  • •What fraction of stocks that screen as "cheap" on P/E actually outperform over the next 3 years?
  • •What is the base rate of successful turnarounds in companies with declining revenue?
  • •How often do management teams that promise margin expansion actually deliver it?
  • •What proportion of IPOs outperform the market over 5 years?

In each case, anchoring on the specific narrative — a compelling turnaround story, a charismatic CEO, a plausible-sounding thesis — without reference to the base rate of similar situations is a recipe for systematic overconfidence.

Good probabilistic thinking starts with the outside view: what happens to companies like this one, in situations like this one, most of the time? Then you update from there, using case-specific evidence — but never abandoning the base rate entirely.

Key Takeaway

Start with the outside view. Before evaluating any specific investment thesis, ask: “What is the base rate of success for situations like this?” Then update from there using Bayesian reasoning. The investors who consistently outperform are those who resist the seduction of the narrative and anchor on frequencies.