Empirical probability, also known as relative frequency, or experimental probability, is the ratio of the number favourable outcomes to the total number of trials , not in a sample space but in an actual sequence of experiments. In a more general sense, empirical probability estimates probabilities from experience an observation. The phrase a posteriori probability has also been used an alternative to emiprical probability or relative frequency. This unusual usage of the phrase is not directly related to Bayesian inference and not to be confused with its equally occasional use to refer to posterior probability, which is something else.