# Probability judgement from samples: accurate estimates and the conjunction fallacy

- Rita Howe,
*University College Dublin*
- Fintan Costello,
*University College Dublin*

## Abstract

This paper investigates a fundamental conflict in the literature
on people's probability estimation. Research on 'perception' of probability
shows that people are accurate in their estimates of probability of various
simple events from samples. Equally, however, a large body of research shows that
people's probability estimates are fundamentally biased, and subject to reliable
and striking fallacies in reasoning. We investigate this conflict in an
experiment that examines the occurrence of the conjunction fallacy in a
probability perception task where people are asked to estimate the probability of
simple and conjunctive events in a presented set of items. We find that people's
probability estimates are accurate, especially for simple events, just as seen in
previous studies. People's estimates also show high rates of occurrence of the
conjunction fallacy. We show how this apparently contradictory result is
consistent with a recent model of probability estimation, the ‘probability
theory plus noise' model.

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