# Defining Probabilities of Events

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## Abstract

Probabilities measure the chance an event will occur. In risk assessments and project evaluations we are not dealing with absolute probabilities, but with relative probabilities (within a portfolio) over a specific time. Generally the time is one year and we use therefore annual probabilities. Do not confuse annual probabilities and frequencies which measure average number of occurrence over a certain time interval. The confusion comes from the fact that the number expressing probability and frequency is very similar, once it goes below say 1/10. Hence a probability of 1/10 has the same number—0.1—as a frequency of 1/10, but they do not mean at all the same conceptually.

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