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:

  1. 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%.
  2. Based on historical evidence, scientists can provide a frequentist probability of experiencing a category 5 hurricane in a given year.


  1. 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.
  2. 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

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