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Chapter 9 • proCess speCifiCations and struCtured deCisions 245

Rules Figure 9.9
Conditions and Actions 1 2 3 4 5 6 7 8 Constructing a decision table for
deciding which catalog to send to
Customer ordered from Fall catalog. Y Y Y Y N N N N customers who order only from
Customer ordered from Christmas catalog. Y Y N N Y Y N N selected catalogs.
Customer ordered from specialty catalog. Y N Y N Y N Y N

Send out this year’s Christmas catalog. X X X X
Send out specialty catalog. X X
Send out both catalogs. X X

taken (A1: send out this year’s Christmas catalog; A2: send out the new specialty catalog; and
A3: send out both catalogs). The resulting decision table has six rows (three conditions and three
actions) and eight columns (two alternatives × two alternatives × two alternatives).
The decision table is now examined to see if it can be reduced. There are no mutually exclu-
sive conditions, so it is not possible to get by with fewer than three condition rows. No rules
allow the combination of actions. It is possible, however, to combine some of the rules, as shown
in Figure 9.10. For instance, Rules 2, 4, 6, and 8 can be combined because they all have two
things in common:

1. They instruct us to send out this year’s Christmas catalog.
2. The alternative for Condition 3 is always N.
It doesn’t matter what the alternatives are for the first two conditions, so it is possible to insert
dashes [—] in place of the Y or N.
The remaining rules—Rules 1, 3, 5, and 7—cannot be reduced to a single rule because two
different actions remain. Instead, Rules 1 and 5 can be combined; likewise, Rules 3 and 7 can be
combined.

Checking for Completeness and Accuracy
Checking over your decision tables for completeness and accuracy is essential. Four main prob-
lems can occur in developing decision tables: incompleteness, impossible situations, contradic-
tions, and redundancy.

Figure 9.10
Rules
Conditions and Actions 1 2 3 4 5 6 7 8 Combining rules to simplify the
customer-catalog decision table.
Customer ordered from Fall catalog. Y Y Y Y N N N N
Customer ordered from Christmas catalog. Y Y N N Y Y N N
Customer ordered from specialty catalog. Y N Y N Y N Y N
Send out this year’s Christmas catalog. X X X X
Send out specialty catalog. X X
Send out both catalogs. X X

Rules
Conditions and Actions 1' 2' 3'

Customer ordered from Fall catalog.
Customer ordered from Christmas catalog. Y N
Customer ordered from specialty catalog. Y N Y
Send out this year’s Christmas catalog. X
Send out specialty catalog. X
Send out both catalogs. X