Certainty factors are numbers that reflect the degree of certainty about a variable's value, an expression's value, or the appropriateness of a rule's action. In the course of reasoning, certainties need to be combined to derive other certainties. Multiple methods exist for combining certainty factors. Both categories come into play for each of the certainty-combining situations. The settings for such controls are chosen to yield certainties that most nearly approximate those of the expert being modeled.
Fuzzy variables are variables that have multiple values.
Each value of a fuzzy variable has a certainty factor. The result is typically multiple values for the consultation's goal variable, each with some reported degree of certainty. It should be kept in mind that an understanding of certainty factors and fuzzy variables is not necessary in order to use an expert system. Furthermore, interesting expert systems can be developed without using such features. However, there are many applications where uncertainty arises.