Decision table testing: columns, collapsing, coverage

Problem

Three tests, a wrong price

You know the discount rule: the promo code gives 10 % and does not apply to sale products; loyal customers get 5 %, on sale products too; discounts do not add up, the larger one applies.

The tester writes three tests: a customer with a code (10 %), a loyal customer without a code (5 %), a customer with nothing (0 %). They pass.

After release a loyal customer enters a code for a product not on sale and gets 15 %. Every condition was tested. The combination where the bug lived was not.

The idea

Every column is a test

In the lesson "When a table is clearer" the decision table was a requirement. Now it is test design: every column is a test case.

	1	2	3	4	5	6	7	8
The code is valid	yes	yes	yes	yes	no	no	no	no
The product is on sale	yes	yes	no	no	yes	yes	no	no
The customer is loyal	yes	no	yes	no	yes	no	yes	no
Discount	?	?	?	10 %	?	?	5 %	0 %

The tests from the start were columns 4, 7 and 8. The bug was in column 3. The coverage of the table is how many of the columns are tested: here 3 of 8.

The full table is easy to build: the conditions become rows, and the columns are all the combinations. Three conditions with "yes" or "no" give 2 × 2 × 2 = 8.

Try it

Columns as tests

Fill in the discount for each of the eight columns. Here the columns of the table are rows: every row is one test case.

The rule: A valid promo code gives 10%, but not on products on sale. A loyal customer gets 5%, on sale products too. Discounts do not add up: the larger one applies.

	Input			Expected	The correct system
#	Valid code	On sale	Loyal	Discount	The correct system

Write the discount as a number: 10 or 10%. An empty result cell is not checked.

Valid code	On sale	Loyal	Discount
yes	yes	yes	5
yes	yes	no	0
yes	no	yes	10
yes	no	no	10
no	yes	yes	5
no	yes	no	0
no	no	yes	5
no	no	no	0

The idea

Collapsing

Look at columns 1 and 5. They differ only in the code, and the result is the same: 5 %. When the product is on sale and the customer is loyal, the code does not matter. The two columns merge into one, and "—" is written in place of the code.

	1+5	2+6	3+4	7	8
The code is valid	—	—	yes	no	no
The product is on sale	yes	yes	no	no	no
The customer is loyal	yes	no	—	yes	no
Discount	5 %	0 %	10 %	5 %	0 %

Eight tests become five. This is collapsing the table.

Its cost: merging says "by the rule this condition does not matter". But the black box does not know whether the code agrees. Columns 3 and 4 merge into "code yes, sale no", and column 3 is exactly where the code gave 15 %. If from the merged column you test only a customer who is not loyal, the bug slips through.

A test cannot be "—": in place of the dash you pick a value. Pick the one where a bug is most likely. That is why for risky rules the full table is tested, and you collapse when tests are expensive and the risk is small.

More precisely, if you are curious

ISTQB CTFL v4.0.1 describes the technique in section 4.2.3 (decision table testing). Conditions and actions are rows, columns are the rules (the combinations). The notation: "T" and "F" for conditions, "—" for a condition that does not matter, "N/A" for an infeasible combination, "X" for an action that takes place. Coverage is the number of tested columns over all feasible ones. Collapsing (minimization) reduces the number of tests, but the syllabus notes that part of the checking is lost that way.

Try it

Merge the columns

Build the collapsed table: at most 5 rows that still catch every bug. Where the lesson has "—", pick a value.

The rule: A valid promo code gives 10%, but not on products on sale. A loyal customer gets 5%, on sale products too. Discounts do not add up: the larger one applies.

	Input			Expected	The correct system
#	Valid code	On sale	Loyal	Discount	The correct system

Write the discount as a number: 10 or 10%. An empty result cell is not checked.

Valid code	On sale	Loyal	Discount
yes	yes	yes	5
yes	yes	no	0
yes	no	yes	10
no	no	yes	5
no	no	no	0

The idea

More than two values

"The code is valid" looks like "yes" or "no". In the shop it is not. The code can be not entered, valid, expired or already used, and each gives the customer something different:

	1	2	3	4	5
A code is entered	no	yes	yes	yes	yes
It has expired	N/A	no	yes	no	yes
It has been used	N/A	no	no	yes	yes
The customer sees	the total without a code	the discount	"The code has expired"	"The code has already been used"	?

Two things show at once.

Impossible combinations. A code that was not entered cannot have expired. Such columns are marked as impossible (N/A) and are not tested.

The empty cell. Column 5: the code has both expired and been used. Which message? The rule does not say. The tester does not decide for the owner. This is a finding, and it came out of the table before there was any code. That is why test design is also a review.

Columns grow fast: one condition with four values and two with two each give 4 × 2 × 2 = 16. With ten "yes or no" conditions it is 1024. Then the rule is split into smaller tables, and the combinations are chosen with another technique, such as pairwise testing later in the module.

Exercise

The tables of the code

1. What does the table say? Four questions the tester finds while writing the table.

1. The code is entered, it has expired and it has already been used. Which message does the customer see?"The code has already been used""The code has expired"It is not said: this is a question for the owner

2. The condition "the code has expired" in the column "no code entered". What goes in the cell?The combination is impossible: the column is removed or marked as impossible"no""yes"

3. The code condition has four values: no code, valid, expired, used. Sale and loyalty are "yes" or "no". How many columns does the full table have?1686

4. The cart is 55 € before the discount and 49.50 € after it. What is the delivery fee?0.00 €4.90 €It is not decided: whether the threshold is checked before or after the discount is an open question

Test

Check yourself

1. The tests are for columns 4, 7 and 8 of the discounts table. What is its coverage?100 %: all three conditions were tested3 of 8 columns3 of 3 conditions

2. What is one test case in a decision table?The whole tableOne column: a combination of conditions and an expected resultOne row: one condition

3. How many columns does the full table have for 4 conditions with "yes" or "no"?8416

4. When can two columns merge?When they are next to each otherWhen both are "yes"When they give the same result and differ in only one condition

5. What does a decision table find besides tests?Combinations the rule says nothing about or contradicts itself onSlow pagesBugs in the code

6. The table has 10 conditions. What do you do?Write 1024 testsSplit the rule into smaller tables, or choose the combinations with another techniqueTest only the first three conditions

Valid code	On sale	Loyal	Discount
yes	yes	yes	5
yes	yes	no	0
yes	no	yes	10
yes	no	no	10
no	yes	yes	5
no	yes	no	0
no	no	yes	5
no	no	no	0

Valid code	On sale	Loyal	Discount
yes	yes	yes	5
yes	yes	no	0
yes	no	yes	10
yes	no	no	10
no	yes	yes	5
no	yes	no	0
no	no	yes	5
no	no	no	0

Decision tables

Cookies

Valid code	On sale	Loyal	Discount
yes	yes	yes	5
yes	yes	no	0
yes	no	yes	10
yes	no	no	10
no	yes	yes	5
no	yes	no	0
no	no	yes	5
no	no	no	0