# Probability

Reference work entry

First Online:

**DOI:**https://doi.org/10.1007/978-3-319-73568-9_228

## Definition

In simple terms, the likelihood that a specified event will occur.

The likelihood can be expressed qualitatively (e.g. a low-to-moderate chance of rain tomorrow) or quantitatively (e.g. a 30% probability of measureable rain in the forecast area on Monday before 3 pm local time). Event likelihood ranges from 0 to 1, meaning that the event is either certain to not occur or certain to occur, respectively. A toss of a fair coin has an equal probability of landing head or tail up. The probability of one outcome, either head or tail, out of two possible outcomes is 1/2 = 0.5. In cases where the outcome can range continuously from zero to 100, as in an analysis of the percentage of volcanic clasts in a conglomerate formation (Fig.
1), the mean and standard deviation values can be used to define a range of expected values and to infer the probability that conglomerate 2 and conglomerate 1 have the same provenance.

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## References

- USGS (2016) Floods: recurrence intervals and 100-year floods (USGS): U.S. Geological Survey The USGS Water Science School. http://water.usgs.gov/edu/100yearflood.html. Accessed Nov 2016

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