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246 part 3 • the analysis proCess

Figure 9.11
Rules
Adding a rule to the customer- Conditions and Actions 1' 2' 3' 4'
catalog decision table changes the
entire table. Customer ordered from Fall catalog.
Customer ordered from Christmas catalog. Y N
Customer ordered from specialty catalog. Y N Y
Customer ordered $50 or more. Y Y Y N
Send out this year’s Christmas catalog. X
Send out specialty catalog. X
Send out both catalogs. X
Do not send out any catalog. X

Ensuring that all conditions, condition alternatives, actions, and action rules are complete is
of utmost importance. Suppose an important condition—if a customer ordered less than $50—
had been left out of the catalog store problem discussed earlier. The whole decision table would
change because a new condition, new set of alternatives, new action, and one or more new action
rules would have to be added. Suppose the rule is: IF the customer did not order more than $50,
THEN do not send any catalogs. A new Rule 4 would be added to the decision table, as shown
in Figure 9.11.
When building decision tables as outlined in the foregoing steps, it is sometimes possible to
set up impossible situations. An example is shown in Figure 9.12. Rule 1 is not feasible, because
a person cannot earn greater than $50,000 per year and less than $2,000 per month at the same
time. The other three rules are valid. The problem went unnoticed because the first condition was
measured in years and the second condition in months.
Contradictions occur when rules suggest different actions but satisfy the same conditions.
The fault could lie with the way the analyst constructed the table or with the information the
analyst received. Contradictions often occur if dashes [—] are incorrectly inserted into the table.
Redundancy occurs when identical sets of alternatives require the exact same action. Figure 9.13
illustrates a contradiction and a redundancy. The analyst has to determine what is correct and
then resolve the contradiction or redundancy.
Decision tables are an important tool in the analysis of structured decisions. One major
advantage of using decision tables over other methods is that tables help the analyst ensure com-
pleteness. When using decision tables, it is also easy to check for possible errors, such as impos-
sible situations, contradictions, and redundancy. Decision table processors, which take the table
as input and provide computer program code as output, are also available.

Decision Trees
Decision trees are used when complex branching occurs in a structured decision process. Trees
are also useful when it is essential to keep a string of decisions in a particular sequence. Although
the decision tree derives its name from natural trees, decision trees are most often drawn on their

Figure 9.12 Rules
Conditions and Actions 1 2 3 4
Checking the decision table for
impossible situations is important.
Salary > $50,000/year Y Y N N
Salary < $2,000/month Y N Y N

Action 1
Action 2

This is an
impossible
situation.