A decision table lists causes and effects in a matrix. Each column represents a unique combination.
Purpose is to structure logic
Cause = condition Effect = action = expected results
Steps to Create a decision table
1. List all causes in the decision table
2. Calculate the number of possible combinations
3. Fill columns with all possible combinations
4. Reduce test combinations
5. Check covered combinations
6. Add effects to the table
Step 1: List all causes
Write down the values the cause/condition can assume
Cluster related causes
Put the most dominating cause first
Put multi valued causes last
Step 2: Calculate combinations
If all causes are simply Y/N values: 2 number of causes
If 1 cause with 3 values and 3 with 2: 3 1 * 2 3 = 24
Or, use the Values column and multiply each value down the column, eg. 3*2*2*2=24
Number of Values to the power of the number of causes with these values
Step 3: Fill columns
Determine Repeating Factor (RF): divide remaining combinations by the number of possible values for that cause
Write RF times the first value, then RF times the next etc. until row is full
Next row, go to 1.
Step 4: Reduce combinations
Find indifferent combinations – place a ‘-’
Join columns where columns are identical
Tip: ensure the effects are the same
Step 5: Check covered combinations
For each column calculate the combinations it represents
A ‘-’ represents as many combinations as the cause has
Multiply for each ‘-’ down the column
Add up total and compare with step 2
Step 6: Add effects to table
Read column by column and determine the effects
One effect can occur in multiple test combinations
Exercise: Specification Create a decision table
A mailing is to be sent out to customers. The content of the mailing is about the current level of discounting and potential levels of discounting. The content is different for different types of customers.
Customer Types A, B and C get a normal letter except Customer Type C, who get a special letter. Any customer with 2 or more current lines or with a credit rating of ‘X’ get a special paragraph added with an offer to subscribe to another level of discounting.
Exercise: possible solution “ 2 or more current lines OR credit rating X”. What if both: AND ? Other customer types? See “O-Other” above. What about non current lines?