Not every frequency format facilitates Bayesian reasoning. Natural frequencies refer to frequency information that results from "natural sampling", which preserves base rate information (e.g., number of drunken drivers when taking a random sample of drivers). This is different from "systematic sampling", in which base rates are fixed a priori (e.g., in scientific experiments). In the latter case it is not possible to infer the posterior probability "p" (drunk | positive test) from comparing the number of drivers who are drunk and test positive compared to the total number of people who get a positive breathalyzer result, because base rate information is not preserved and must be explicitly re-introduced using Bayes' theorem.