Decision tables

Decision Tables
A decision table is a table composed of rows and columns, separated into four separate quadrants.
Conditions Condition Alternatives
Actions Action Entries
The upper left quadrant contains the conditions. The upper right quadrant contains the condition rules ofr alternatives. The lower left quadrant contains the actions to be taken and the lower right quadrant contains the action rules.
Developing Decision Tables
In order to build decision tables, you need to determine the maximum size of the table, eliminate any impossible situations, inconsistencies, or redundancies, and simplify the table as much as possible. The following steps provide offer some guidelines to developing decision tables:
1. Determine the number of conditions that may affect the decision. Combine rows that overlap, for example, conditions that are mutually exclusive. The number of conditions becomes the number of rows in the top half of the decision table.
2. Determine the number of possible actions that can be taken. This becomes the number of rows in the lower half of the decision table.
3. Determine the number of condition alternatives for each condition. In the simplest form of decision table, there would be two alternatives (Y or N) for each condition. In an extended-entry table, there may be many alternatives for each condition.
4. Calculate the maximum number of columns in the decision table by multiplying the number of alternatives for each condition. If there were four conditions and two alternatives (Y or N) for each of the conditions, there would be sixteen possibilities as follows:
Condition 1: x 2 alternatives
Condition 2: x 2 alternatives
Condition 3: x 2 alternatives
Condition 4: x 2 alternatives
16 possibilities
5. Fill in the condition alternatives. Start with the first condition and divide the number of columns by the number of alternatives for that condition. In the foregoing example, there are sixteen columns and two alternatives (Y and N), so sixteen divided by two is eight. Then choose one of the alternatives and write Y in all of the eight columns. Finish by writing N in the remaining eight columns as follows:
Condition 1 YYYYYYYYNNNNNNNN
Repeat this for each condition using a subset of the table:
Y Y Y Y Y Y Y Y N N N N N N N N
Y Y Y Y  N N N N
Y Y N N
Y  N
and continue the pattern for each condition:
Condition 1  Y Y Y Y Y Y Y Y N N N N N N N N
Condition 2  Y Y Y Y N N N N  Y Y Y Y N N N N
Condition 3  Y Y N N  Y Y N N Y Y N N Y Y N N
Condition 4  Y N  Y N Y N Y N Y N Y N Y N Y N