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satisfied, the false branch is taken and its rule evaluated. When a terminal node is
reached, if its rule is satisfied, then its conclusion is taken; if its rule is not satisfied,
then the conclusion of the last rule satisfied on the path to that node is taken. For
terminal nodes on a true branch, if its rule is not satisfied then the last rule satisfied
will always be that of its parent (since it must have evaluated to true for the terminal
node to be evaluated).
This tree traversal gives RDR implied locality – a rule on an exception branch is
tested for applicability only if its parent (next-general) rule is also applicable. This
feature provides the fundamental benefit of RDR: general rules are defined first, and
refinements to those rules added later as the need arises or as knowledge about the
domain grows. Thus, there is always a working rule set that extends over time.
For example, the rule tree in Fig. 4.1 represents the following illustrative set of
(somewhat artificial) business rules:
1. Node 0: By default, assess a claim
2. Node 1: An exception to the node 0 rule is that for those cases where the claim
amount is greater than $10,000, the claim must be referred to a manager (since
the condition of node 0 is always satisfied, the condition of node 1 will always
be evaluated)
3. Node 3: An exception to the node 1 rule is that cases where the claim amount
is greater than $10,000 may be assessed by an assessor of at least Senior role
(satisfied node 1)
4. Node 6: An exception to the node 3 rule is that those cases where the claim
amount is greater than $50,000 must always be referred to a manager (regardless
of the rank of available assessors (satisfied node 3)
5. Node 2: If the claim amount is less than $10,000 (and so the condition of node
1 was not satisfied), then if the claimant has already made a claim in the current
year it must be investigated
6. Node 4: An exception to the node 2 rule is that if the claim amount is also less
than $3,000, simply assess the claim (i.e., satisfied node 2, but the amount is too
trivial to warrant an investigation)
7. Node 5: An exception to the node 4 rule is that for those cases where the claimant
status is set to “suspect,” the claim should always be investigated (satisfied
node 4)
8. Node 7: If the claim amount is less than or equal to $10,000 (unsatisfied node 1)
and there has been a claim from this claimant in the current year (satisfied
node 2) and the claim amount is also greater than or equal to $3,000 (unsatisfied
node 4) and the claim is for storm damage, then the claim should be escalated for
express payment
If the conclusion returned is found to be unsuitable for a particular case instance –
that is, while the conclusion was correct based on the current rule set, the circum-
stances of the case instance make the conclusion an inappropriate choice – a new
rule is formulated that defines the contextual circumstances of the instance and is
added as a new leaf node using the following algorithm: