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Decision Tables

Condition entry is used for specifying which condition is being analyzed. Action stub shows the various actions taken against different conditions.

And action entry is used to find out which action is taken corresponding to a particular set of conditions.

The steps to be taken for a certain possible condition are listed by action statements. Action entries display what specific actions to be undertaken when selected conditions or combinations of conditions are true. At times notes are added below the table to indicate when to use the table or to distinguish it from other decisions tables.

The right side columns of the table link conditions and actions and form the decision rules hence they state the conditions that must be fulfilled for a particular set of actions to be taken. In the decision trees, a fixed ordered sequence is followed in which conditions are examined. But this is not the case here as the decision rule incorporates all the required conditions, which must be true.

Developing Decision Tables

Before describing the steps involved in building the decision table it is important to take a note of few important points. Every decision should be given a name and the logic of the decision table is independent of the sequence in which condition rules are written but the action takes place in the order in which events occur. Wherever possible, duplication of terms and meaning should be avoided and only the standardized language must be used.

The steps of building the concerned tables are given below.

1. Firstly figure out the most essential factors to be considered in making a decision.
   
   This will identify the conditions involved in the decision. Only those conditions should be selected which have the potential to either occur or not but partial occurrences are not permissible.

2. Determine the most possible steps that can take place under varying conditions and not just under current condition. This step will identify the actions.

3. Calculate all the possible combinations of conditions.
   
   For every N number of conditions there are 2*2*2…. (N times) combinations to be considered.

4. Fill the decision rules in the table.
   
   Entries in a decision table are filled as Y/N and action entries are generally marked as "X". For the conditions that are immaterial a hyphen "-" is generally put. Decision table is further simplified by eliminating and consolidating certain rules. Impossible rules are eliminated. There are certain conditions whose values do not affect the decision and always result in the same action. These rules can be consolidated into a single rule.

Example: Consider the recruitment policy of ABC Software Ltd.

It the applicant is a BE then recruit otherwise not. If the person is from Computer Science, put him/her in the software development department and if the person is from non-computer science background put him/her in HR department. If the Person is from Computer Science and having experience equal to or greater than three years, take him/her as Team leader and if the experience is less than that then take the person as Team member. If the person recruited is from non Computer Science background, having experience less than three years, make him/her Management Trainee otherwise Manager.

Decision table-Discount Policy

The first decision table for the problem stated above can be drawn as shown in the figure below.

This table can further be refined by combining condition entries 2, 4, 6, and 8. The simplified table is displayed in the figure below.

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