Incorrect Answers

(6th-August-2020)

• An incorrect answer is an answer that has been proved yet is false in the intended interpretation. It is also called a false-positive error. An incorrect answer can only be produced by a sound proof procedure if an incorrect definite clause was used in the proof.

• Assume that whoever is debugging the knowledge base, such as a domain expert or a user, knows the intended interpretation of the symbols of the language and can determine whether a particular proposition is true or false in the intended interpretation. The person does not have to know how the answer was derived. To debug an incorrect answer, a domain expert needs only to answer yes-or-no questions.

• Suppose there is an atom g that was proved yet is false in the intended interpretation. Then there must be a rule g←a1∧...∧ak in the knowledge base that was used to prove g. Either

• one of the ai is false in the intended interpretation, in which case it can be debugged in the same way, or

• all of the ai are true in the intended interpretation. In this case, the definite clause g←a1∧...∧ak must be incorrect.

• This leads to an algorithm, presented in Figure 5.6, to debug a knowledge base when an atom that is false in the intended interpretation is derived.