Our classifier will use rules from the program RGEN. These are rules with a conjunctive precondition and a single decision value. In addition, they contain the number of objects matching the precondition, and the percentage of these objects matching the decision as well, called accuracy or validity. The rules are thus of the following form where means ``and'':
Rule 13 : A=3 B=4 D=5 with probability 0.75. Preconditions match 40 objects.
The rules also may have so called blocks. These give conditions for when a rule should not be used. A block for the rule above could be:
Block 1: c=4 block rule 13.
Another possibility would be to include this block in the rule itself, by writing: A=3 B=4 D=5 ...
The rules are generated at specific nodes in the lattice, as described
in Chapter 
. The information about which rules are
created at which points in the lattice, is contained in the rule list. The
information given is which attributes were used at that node and
a rule base number which is a sequential numbering of the nodes in
the order they were traversed.
If the information about the lattice is dropped, our rules may be regarded as corresponding to facts and defaults in the framework of Poole's Theorist. The defaults are our usual rules, and the facts are given indirectly by the blocks: If a rule has a block, this is because a fact exists, which then overrides the default normally used.