# What makes intensional estimates of probabilities inconsistent?

- Sangeet Khemlani,
*Princeton University*
- Max Lotstein,
*Princeton University*
- Phil Johnson-Laird,
*Princeton University*

## Abstract

Individuals are happy to make estimates of the probabilities of
unique events. Such estimates have no right or wrong answers, but when they
suffice to determine the joint probability distribution, they should at least be
consistent, yielding one that sums to unity. Mental model theory predicts two
main sources of inconsistency: the need to estimate the probabilities that events
do not happen, and the need to estimate conditional probabilities as opposed,
say, to conjunctive probabilities. Experiments 1 and 2 corroborated the first
prediction: when the number of estimates of non-events increased for a problem,
so did the degree of overall inconsistency. Experiment 3 corroborated the second
prediction: when the number of estimates of conditional probabilities increased,
the degree of overall inconsistency was larger as well.

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