### Definition(s)

#### Frequentist Probability

Definition: interpretation or estimate of probability as the long-run frequency of the occurrence of an event as estimated by historical observation or experimental trials.

**Sample Usage:**

- Based on empirical evidence from repeated experimental trials, the frequentist probability of getting a three when rolling a fair six-sided die is 1/6 or 16.7%.
- Based on historical evidence, scientists can provide a frequentist probability of experiencing a category 5 hurricane in a given year.

**Annotation:**

- Within the frequentist probability interpretation, precise estimation of new or rarely occurring events, such as the probability of a catastrophic terrorist attack, is generally not possible.
- Frequentist probabilities generally do not incorporate ―degree of belief‖ information, such as certain types of intelligence information.

*Source: DHS Risk Lexicon, U.S. Department of Homeland Security, 2010 Edition. September 2010 Regulatory Guidance*