Decision Table and Collapsed Decision Table

A decision table is a good way to deal with combinations of things (e.g. inputs). This technique is sometimes also referred to as a ’cause-effect’ table.

How to Use decision tables for test designing?

Technique followed for decision table testing are follows:

•  Analyze the given test inputs or requirements an list out the various Conditions in the decision table.
• Calculate the number of possible combinations called Rules. Rules is usually calculated as 2 to the power of condition (2^condition). 
• Fill columns of the decision table with all possible combinations (Rules)
• For each of the combination of values, find out the Action or Expected result.

Let us consider an example to construct decision table:

Q: For persons in the age group 16-65 (both inclusive) tax payment have to be calculated. A person earning less than £20,000 will pay 20% taxes, otherwise they pay 50% taxes. If the person has children he/she will receive a tax reduction of 10%.

Let us first identify conditions and action in the given example,

Condition	Values
Age	16 – 65 and  >16/>65
Earning	<20,000 and >20,000
Children	Yes and No

Action

Tax 20%

Tax 50%

Tax Reduction 10%

No Tax Calculation

In order to construct decision table we need to know number of columns/ test set that is derived from formula explained earlier i.e., 2^condition so in our example it is 2^3 = 8 test sets.

Lets construct Decision Table:

Condition	Numbers	1	2	3	4	5	6	7	8
	Age 16 - 65
	Earning < 20, 000
	Children = Yes
Action	Tax 20%
	Tax 50%
	Tax Reduction 10 %
	No Tax Calculation
	Result

as we know we need 8 test set hence we have 8 columns which are numbered. If you look at condition here we have taken only positive set as when you go on filling decision table we fill it with either 1 or 0. where 1 represents positive set and 0 represents negative set. For example in case of condition Children 1 = Yes and 0 = No

Easily way to fill in Decision table:

1st Row = start with 1 and fill in test set / 2 columns with it. i.e., 8/2 =4 so , first 4 columns with 1 and next 4 column with 0

2nd Row = start with 1 and fill in test set/4 with it. i.e., 8/4 = 2 so, first 2 column with 1 and second 2 columns with zero and repeat same for rest of 4 columns

3rd Row = start with 1 and fill in test set/8 with it. i.e., 8/8 = 1 so first column with 1 and second column with 0 and repeat same for rest of the columns.

Completed table should look like this: 

Condition	Numbers	1	2	3	4	5	6	7	8
	Age 16 - 65	1	1	1	1	0	0	0	0
	Earning < 20, 000	1	1	0	0	1	1	0	0
	Children = Yes	1	0	1	0	1	0	1	0
Action	Tax 20%
	Tax 50%
	Tax Reduction 10 %
	No Tax Calculation
	Result

Now lets start marking actions in the table, 

1st column: A Person with age 16 - 65 and earning less than 20,000 and has children so he has to pay 20% tax because he has child he gets 10 % tax deduction = 10% Tax

2nd Column:A  Person with age 16- 65 , earning < 20,000 and has no children hence get does not get any deduction and has to pay 20% tax.

3rd Column: A Person with age 16 - 65 and earns > 20, 000 and has children hence he has to pay 50 % tax because he has child he gets 10 % deduction  = 40% tax

Now, you just go on to fill in rest of columns. Completed table looks like this:

Condition	Numbers	1	2	3	4	5	6	7	8
	Age 16 - 65	1	1	1	1	0	0	0	0
	Earning < 20, 000	1	1	0	0	1	1	0	0
	Children = Yes	1	0	1	0	1	0	1	0
Action	Tax 20%	X	X	-	-	-	-	-	-
	Tax 50%	-	-	X	X	-	-	-	-
	Tax Reduction 10 %	X	-	X	-	-	-	-	-
	No Tax Calculation	-	-	-	-	X	X	X	X
	Result	10%	20%	40%	50%	0%	0%	0%	0%

Now I would like to talk about Collapsed Decision Table, if for two test columns only one condition differs and resulting actions are same these columns can be joined or in other words delete likely non - relevant test cases.

Rules for applying collapsing decision table:

Step 1: Compare column 1-2, 3-4, 5-6, 7-8
Step 2: Compare column 1-3, 2-4, 5-7, 6-8
Step 3: Compare column 1-5, 2-6, 3-7, 4-8

Things to remember:

1. Always delete column on the right
2. Do not rename the columns after deletion. 

Let us apply concept of collapsed decision table to above decision table we just constructed

Step 1: Applying this role to the table above,

Column 1- 2 = not same action
Column 3-4 = not same action
Column 5-6  = are same in terms of action with only condition differing i.e., children hence we can delete column 6 ( column on right ).
Column 7-8 = are same in terms of action with only condition differing i.e., children hence we can delete column 8 ( column on right ).

After applying step 1 table looks like below:

Condition	Numbers	1	2	3	4	5	7
	Age 16 - 65	1	1	1	1	0	0
	Earning < 20, 000	1	1	0	0	1	0
	Children = Yes	1	0	1	0	~	~
Action	Tax 20%	X	X	-	-	-	-
	Tax 50%	-	-	X	X	-	-
	Tax Reduction 10 %	X	-	X	-	-	-
	No Tax Calculation	-	-	-	-	X	X
	Result	10%	20%	40%	50%	0%	0%

Step 2: Applying this role to the table above,

Column 1-3 = not same action
Column 2-4 = not same action
Column 5-7  = are same in terms of action with only condition differing i.e.,earning hence we can delete column 7 ( column on right ).
Column 6-8 = as there is no column to compare, we just ignore this

After applying step 2 table looks like below:

Condition	Numbers	1	2	3	4	5
	Age 16 - 65	1	1	1	1	0
	Earning < 20, 000	1	1	0	0	~
	Children = Yes	1	0	1	0	~
Action	Tax 20%	X	X	-	-	-
	Tax 50%	-	-	X	X	-
	Tax Reduction 10 %	X	-	X	-	-
	No Tax Calculation	-	-	-	-	X
	Result	10%	20%	40%	50%	0%

Step 3: Applying this role to the table above,

Column 1-5 = not same action
Column 2-6 = as there is no column 6 to compare, we just ignore this
Column 3-7  = as there is no column 7 to compare, we just ignore this
Column 4-8 = as there is no column 8 to compare, we just ignore this

So in our example collapsed decision table is:

Condition	Numbers	1	2	3	4	5
	Age 16 - 65	1	1	1	1	0
	Earning < 20, 000	1	1	0	0	~
	Children = Yes	1	0	1	0	~
Action	Tax 20%	X	X	-	-	-
	Tax 50%	-	-	X	X	-
	Tax Reduction 10 %	X	-	X	-	-
	No Tax Calculation	-	-	-	-	X
	Result	10%	20%	40%	50%	0%

I hope, now concept of Decision table and Collapsed decision table is clear. If you have any question you can reach out to me. Please leave your questions in the comments.