Statistical Regularity has motivated the development of the relative frequency concept of probability. Most of the procedures commonly used to make statistical estimates or tests were developed by statisticians who used this concept exclusively. They are usually called frequentists. This school is often associated with the names of [Jerzy Neyman] and [Egon Pearson] who described the logic of statistical hypothesis testing.
Prior to and during the 1950's and 1960's, there was a debate among statisticians which featured the frequentists versus the Bayesians. The former insisted that statistical procedures only made sense when one uses the relative frequency concept. The Bayesians supported the use of degrees of belief as a basis for a new form of statistical practice.