In a series of observations, or *trials*, the **relative frequency** of occurrence of an event *E* is calculated as the number of times the event *E* happened over the total number of observations made. The **relative frequency density** of occurrence of an event is the relative frequency of *E* divided by the size of the bin used to classify *E*.

The **limiting relative frequency** of an event over a long series of trials is the conceptual foundation of the frequency interpretation of probability. In this framework, it is assumed that as the length of the series increases without bound, the fraction of the experiments in which we observe the event will stabilize. This interpretation is often contrasted with Bayesian probability.