How to Dramatically Reduce the Number of Test Cases Combinations Using Pairwise Testing

What is Pairwise Testing and How It is Effective Test Design Technique for Finding Defects:

In this article we are going to learn about a ‘Combinatorial Testing’ technique called ‘Pairwise Testing’ also known as ‘All-Pairs Testing’.

Smart testing is the need of the hour. 90% of the times system testing team has to work with tight schedules. So test design techniques should be very effective for maximum test coverage and high defect yield rate.

what is pairwise testing

Pairwise Testing is a test design technique that delivers hundred percent test coverage.

The output of a software application depends on many factors e.g. input parameters, state variables and environment configurations. Techniques like boundary value analysis and equivalence partitioning can be useful to identify the possible values for individual factors. But it is impractical to test all possible combinations of values for all those factors. So instead a subset of combinations is generated to satisfy all factors.

Definition: What is Pairwise Testing?

ISTQB defines All-Pairs Testing (or Pairwise Testing) as: A black-box test design technique in which test cases are designed to execute all possible discrete combinations of each pair of input parameters.

All-Pairs technique is very helpful for designing tests for applications involving multiple parameters. Tests are designed such that for each pair of input parameters to a system, there are all possible discrete combinations of those parameters. The test suite covers all combinations; therefore it is not exhaustive yet very effective in finding bugs.

Let us learn how to apply Pairwise Testing by this example.

Pairwise Testing Example

Car Ordering Application:

  • The car ordering application allows for Buying and Selling cars. It should support trading in Delhi and Mumbai.
  • The application should have a registration numbers, may be valid or invalid. It should allow the trade of following cars: BMW, Audi, and Mercedes.
  • Two types of booking can be done: E-booking and In Store.
  • Orders can be placed only during trading hours.

Step #1: Let’s list down the variables involved.

1) Order category
a. Buy
b. Sell

2) Location
a. Delhi
b. Mumbai

3) Car brand
a. BMW
b. Audi
c. Mercedes

4) Registration numbers
a. Valid (5000)
b. Invalid

5) Order type
a. E-Booking
b. In store

6) Order time
a. Working hours
b. Non-working hours

If we want to test all possible valid combinations:
= 2 X 2 X 3 X 5000 X 2 X 2
= 240000  Valid test cases combinations :(

There are also an infinite number of invalid combinations.

Step #2: Let’s simplify

– Use a smart representative sample.
– Use groups and boundaries, even when data is non-discrete.
– Reduce Registration Number to Two

  1. Valid registration number
  2. Invalid registration number

Now let’s calculate number of possible combinations
= 2 X 2 X 3 X 2 X 2 X 2
= 96

Step #3: Arranging variables and values involved.

When we arrange variables and values involved, it looks something like this.

all-pairs testing example 1

Now order the variables so that the one with the most number of values is first and the least is last.

all-pairs testing example 2

Step #4: Arrange variables to create test suite

Let’s start filling in the table column by column. Initially table should look something like this. The three values of Product (variable having highest number of values) should be written two times each (two is the number of values of next highest variable i.e. Order category).

all-pairs testing example 3

The Order Category column has two values. That’s how many times we need to insert the values of the first column, Product.


all-pairs testing example 4

For each set of values in column 1, we put both values of column 2. Repeat the same for column 3.

all-pairs testing example 5

We have a Buy and Delhi, but wait – there’s no Buy and Mumbai. We have a Sell and Mumbai, but there’s no Sell and Delhi. Let’s swap around the values in the second set in the third column.

all-pairs testing example 6

This looks much better!

We will repeat the same steps for column 3 and 4.

all-pairs testing example 7

When columns 3 and 4 are compared, each value in column 3 has both the values of column 4. But when you compare the 2nd and 4th column, we have Buy and Valid & Sell and Invalid .i.e. Buy does not have ‘Invalid’ and Sell does not have ‘Valid’. Hence we need to interchange the last set of values in the 4th column.

all-pairs testing example 8

Column 6 (Order time) is problematic. We are missing Buy/Non-working hours and Sell/Working hours. We can’t fit our missing pairs by swapping around values as we already swapped all the rows if we swap now we may miss other possible pairs which are already sorted. So, we add two more test cases that contain these pairs. Hence, the blank rows!

all-pairs testing example 9

Now we will fill in the empty cells as we desire, because the other variable values are purely arbitrary (or Don’t Cares ~).

all-pairs testing example 10

Hurray! All pairs in 8 cases, instead of all combinations in 96!

Hence, we saw how efficient All-pairs technique of test design is. There stands a good chance of finding bugs and it is fun and powerful.

Pairwise testing technique has some limitations as well.

  • It fails when the values selected for testing are incorrect.
  • It fails when highly probable combinations get too little attention.
  • It fails when interactions between the variables are not understood well.

Pairwise Testing Tools:

Tools are available that applies all-pairs testing technique that facilitates us to effectively automate the Test Case Design process by generating a compact set of parameter value choices as the desired Test Cases. Some well-known tools from the industry are:

  • PICT‘Pairwise Independent Combinatorial Testing’, provided by Microsoft Corp.
  • IBM FoCuS‘Functional Coverage Unified Solution’, provided by IBM.
  • ACTS ‘Advanced Combinatorial Testing System’, provided by NIST, an agency of the US Government.
  • Hexawise
  • Jenny
  • Pairwiser by Inductive AS
  • VPTag free Pairwise Testing Tool

Conclusion:

Pairwise testing technique can dramatically reduce the number of combinations to be covered but remains very effective in terms of fault detection. It is indeed a smart test design technique that guarantees a win-win situation for both test effort and test effectiveness.

During the Test planning phase of software testing, Pairwise testing technique should always be taken into consideration. Either we are doing it manually or using any tool to generate test cases, it becomes a necessary component of the test plan because it in turn affects Test estimation.




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14 comments ↓

#1 Aniket

kudos for the great example. do you have more examples to share?

#2 Meg

Clear explanation on how pairwise combinations are created. Thanks!

#3 Shraddha

Gr8 explanation and nice information
Thanks!

#4 Sunil

Nice explanation. Another example would be good!

#5 Vinay HN

Nice and clear explanation.. Thumbs up..:)

#6 Binu

Really has nice concept reducing the test cases….Next Pairwise.

#7 Gaurav Khurana

Good to know that pairwise testing is so much important that few companies have created tool for it.

what kind of things these tools have provided apart from combination

#8 Nikolay Advolodkin

Really useful article. I enjoyed the read :)

#9 Prathiba

Nice article. Can you explain anyone of the pairwise testing tool widely using across the industry.

#10 Peter

If you swap buy/sell in 2nd column, you will not to add rows in step 6, right? So why don’t you do that?

#11 Vamsi

Simply and clear, It explains the concept in deep. Thank you:)

#12 Ashwini

thanks for this explanation, its useful.
what is concept of 1-wise and 2-wise pair testing? how this will be calculated ?

#13 Sam

Why don’t we have 9th test case for Buy – invalid – non working hours

#14 Rupali

Hi,

i am unable to download Advanced Combinatorial Testing System (ACTS) tool. Can some one help me out ? Actually i am looking for pairwise testing open source tool

Thanks and Regrads,
Rupali.

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