Mutation Testing – Learn This Interesting Testing Technique Quickly with a Simple Example

Sci-Fi fans: Don’t get your hopes up. This isn’t that kind of mutation we are talking about :)

What is Mutation Testing?

Mutation testing is a fault based testing technique where variations of a software program are subjected to the test data-set. This is done to determine the effectiveness of the test set in isolating the deviations.

It sounds a little complicated, isn’t it? 


Let’s learn from an example

Say, there is a hospital site that lets new users register. It reads the Date of birth or age of the patient. If it is greater than 14, assigns a general physician as their main doctor.  To do so, it invokes the ‘general physician’ function that finds the available doctor.

Now, there might be other functionality. Maybe, patients below 13 get assigned to a pediatrician and so on. But we will only take the age-over-14 case.

This is what the code might look like:

1) Read Age
2) If age>14
3) Doctor= General Physician()
4) End if

Please note that the above lines of code are not specific to any programming language and won’t run. It is just hypothetical.

As a tester, if my data-set is 14, 15, 0, 13 – some random numbers.

The target is to check if the data-set of the 4 values (14, 15, 0, and 3) is adequate to identify all possible problems with this code.

Also read => Tips to design test data before executing your test cases

How does Mutation Testing achieve this?

First and foremost, you create mutants- variations of the program. A mutant is nothing but a program that is written as a deviation. It contains a self-seeded fault.

Examples are:

  • Arithmetic operator replacement
  • Logical connector replacement
  • Statement removal
  • Relational operator replacement
  • Absolute value insertion, etc.

These replacements are also called ‘Mutation Operators.’

Let me show you examples:

Mutant #1: Relational operator replacement

1) Read Age
2) If age<14 ‘Changing the > with <’
3) Doctor= General Physician()
4) End if

Mutant #2:

1) Read Age
2) If age=14 ‘Changing the > with =’
3) Doctor= General Physician()
4) End if

Mutant #3:

1) Read Age
2) If age>=14 ‘Changing the > with >=’
3) Doctor= General Physician()
4) End if

Mutant #4:

1) Read Age
2) If age<=14 ‘Changing the > with <=’
3) Doctor= General Physician()
4) End if

Mutant #5: Statement Removal

1) Read Age
2) If age=14
3) ‘remove the doctor assignment statement’
4) End if

Mutant #6: Absolute Value Insertion

1) Read Age
2) If age>14
3) Doctor= Mr.X (Absolute value insertion- let’s say X is a pediatrician)
4) End if

Mutant #7: Incorrect syntax

1) Read Age
2) If age%%14 (incorrect syntax)
3) Doctor=General Physician()
4) End if

Mutant #8: Does the same thing as the original test


1) Read Age
2) If age> 14 & age>14 ‘means the same thing as age>14’
3) Doctor= General Physician()
4) End if

Once, all the mutants are created. They are subjected to the test data-set. Our set is 14, 15, 0 and 13. Which of these mutants will our data-set find?

Find out in the below table:

(Click on image for an enlarged view)


As you can see our data value 14 finds failures when it runs against, Mutant 2, 3 and 4. Or, 14 kills mutants 2, 3 & 4. But, it is ineffective against, 1, 6 and 8.

If your data-set kills all mutants, it is effective. Otherwise, include more or better test data. It is not necessary for the each value in the data-set to kill all mutants. But together, they should kill all. For example: 14 kills 2, 3 and 4. 15 kills 1, 2 and 4. And, so on.

What about 5, 7, and 8?

Mutant #5 – is the program instance that will fail irrespective of any data value you give. This is because it will not do any programming for both valid and invalid values.

Mutant #7 will be a compile error. Or in the case of a scripting language an error that will prevent execution.

Mutant #8 is the same thing as the main program.

As you can see, the above mutants are not useful at all.

Therefore, mutants to avoid are:

  • Syntactically incorrect/‘Still-Born’ mutants. : You need syntactically correct mutants ONLY. Example: Mutant 7
  • Equivalent Mutants: The ones that do the exact same thing as the original program. Example: Mutant 8.
  • Trivial Mutant: Can be killed by any data-set. Example: Mutant 5

Points to note:

  • The number of mutants, even for a small program, can be many. It is a finite number, but still very large. Due to this, a subset of mutants is usually used. It is common to choose the mutants randomly.
  • The mutation operators listed above is not an exhaustive list. There can be many other variations. I have oversimplified the concept for easy understanding.
  • The mutation operators also differ with programming languages, design, and specifications of the application.
  • If there are some mutants alive at the end of the test. It means either it is an invalid mutant (like 5, 7 and 8) or the data-set was inadequate. If it is the later one, go back and change it.
  • Mutation Test is a structural, white-box and unit testing method. It uses fault-injection or fault-seeding to generate its mutants.
  • There are many unit testing frameworks and tools that aid for automatic mutation testing. Some of them are:
    • Jester for JUnit.
    • Pester for Python
    • MuClipse for eclipse, etc.

Are you thinking, if it takes this much effort, what is going to happen when I have to test large samples of code?

Mutation testing relies on two things:

  • Competent Programmer Assumption: If a programmer is competent enough to write and test a small piece of code, he will be good at writing larger programs too.
  • Coupling effect Assumption: If 2 units are combined to form a program and each one is good in itself, then the combination is going to be good too.

So, it focuses on the smallest unit of code and places its faith in the programmer’s skill to scale mutation testing to larger programs.

In conclusion:

I hope you have enjoyed reading and learning about an interesting testing technique- Mutation Testing.

About the author: This article is written by STH team member Swati S.

Please share your comments, questions and thoughts below.


#1 Rajeshwari

Good explanation about Mutation Testing

#2 Hastings

You’ve left no stone upturned. Rich explanation. Thanks.

#3 Sandeep

Good Explanation about Mutation Testing. keep it on.
Very useful as well as better Knowledge. :)

#4 Gbemi

This is instructive.
Good one

#5 Swati Seela

@all: Thank you!

#6 Darshan Satish

Very new info, Thanks :)

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