The number of occurrences of a particular event, E, divided by the total number of observed events. An event is a particular instance of a class of observations, such as the result of the throw of dice, the recording of a man's height, or the survival of a patient given a particular treatment. Relative frequency should be distinguished from probability. For example, the probability that a fair coin when tossed lands heads up is 0.5, whereas the relative frequency in a particular run of 100 tosses might be 47/100 or 0.47.
The set of relative frequencies for all the events that are possible is called a frequency distribution. It may be displayed graphically as a histogram.