72 CHAPTER3 Output: Knowledge Representation it must be in class no--a form of closed-world assumption. If this is the case, rules cannot conflict and there is no ambiguity in rule interpretation: Any interpretation strategy will give the same result. Such a set of rules can be written as a logic expression in what is called disjunctive normal form: that is, as a disjunction (OR) of conjunctive (AND) conditions. It is this simple special case that seduces people into assuming that rules are very easy to deal with, for here each rule really does operate as a new, independent piece of information that contributes in a straightforward way to the disjunction. Unfor- tunately, it only applies to Boolean outcomes and requires the closed-world assump- tion, and both these constraints are unrealistic in most practical situations. Machine learning algorithms that generate rules invariably produce ordered rule sets in multi- class situations, and this sacrifices any possibility of modularity because the order of execution is critical. AssociationRules Association rules are no different from classification rules except that they can