In many situations it is not possible to answer a question with a Yes or No. In the Decision Maker expert system the user may specify "UNK" for unknown by using the CF (Certainty Factor) function. The expert system will then use fuzzy logic in searching for efficient method(s).
In the above example. The user has answered two questions with a degree of certainty and is in the process of answering a third. A CF of 100 is the same as answering "YES" to the question.
http://www.eskimo.com/~mighetto/lscf.htm - last update September 21, 1996