# A model of conditional probability judgment

- Fintan Costello,
*University College Dublin*
- Paul Watts,
*National University of Ireland Maynooth*

## Abstract

A standard view in cognitive psychology is that people estimate
probabilities using heuristics that do not follow probability theory. We
describe a model of probability estimation where people do follow probability
theory in estimation, but are subject to random error or noise. This model
predicts that people's conditional probability estimates will agree closely with
probability theory for certain noise-cancelling expressions, but deviate from
probability theory for other expressions. We describe an experiment which
strongly confirms these predictions, suggesting that people estimate conditional
probabilities in a way that follows standard probability theory, but is subject
to the biasing effects of random noise.

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