Numbers that correspond to the way humans encountered information before the invention of probability theory. Unlike probabilities and relative frequencies, they are raw observations that have not been normalized with respect to the base rates of the event in question.

For instance, a physician has observed 100 persons, 10 of whom show a new disease. Of these 10 persons, 8 show a symptom, whereas 4 of the 90 without the disease also show the symptom. Breaking these 100 cases down into four numbers:

- disease and symptom 8

- disease and no symptom 2

- no disease and symptom 4

- no disease and no symptom 86

This results in four natural frequencies 8, 2, 4, and 86; natural frequencies help to make sound conclusions.

One of the three major interpretations of

The probability of an event is defined as its relative frequency in a reference class. Relative frequencies are constrained to repeated events that can be observed in large numbers.

Adapted from Bandolier. EBM glossary. Internet. Accessed on June 3, 2016.