# Mutation Testing in English: Common Mistakes to Avoid (2026) > A software testing technique where small intentional changes (mutations) are introduced into code to verify that your test suite can detect these bugs. - **Term:** mutation testing - **Part of speech:** noun - **Pronunciation:** /mjuːˈteɪʃən ˈtɛstɪŋ/ - **English level (CEFR):** C1 - **Register:** Formal, Both - **Region:** US and UK - **Topic:** [Code and Git terms](https://app.speaktechenglish.com/learn/topics/software-development-terms.md) - **Lesson:** https://app.speaktechenglish.com/learn/mutation-testing - **Last updated:** 2026-10-27 ## Meaning A software testing technique where small intentional changes (mutations) are introduced into code to verify that your test suite can detect these bugs. It measures how effective your tests are at catching real defects. Mutation testing is an advanced quality assurance practice used to evaluate the quality and effectiveness of your test suite. Instead of just checking if tests pass, it deliberately introduces small bugs into the code to see if your tests will fail. These small changes might be changing a greater-than operator to a less-than operator, or modifying a return value. If your tests catch these mutations, it shows your tests are robust; if not, it reveals gaps in your test coverage. In real tech workplaces, teams use mutation testing tools like PIT (for Java), Stryker (for JavaScript), or Mutmut (for Python) to assess test quality before major releases. This is different from code coverage metrics, which only measure whether code is executed by tests, not whether tests actually verify correct behavior. Teams often discuss mutation testing scores in sprint retrospectives or during code reviews when deciding if a test suite is sufficient for production. ## When you hear it Our mutation testing score is only 60 percent, which means our tests are not catching enough potential bugs. ## Examples - **in a standup:** "I ran mutation testing on the payment module yesterday and found that our tests were not catching edge cases with decimal values." - **in a code review:** "This test looks good, but let's add mutation testing to ensure we're actually validating the business logic and not just happy paths." - **in a team meeting:** "Our mutation testing score improved from 65 percent to 82 percent after we added those extra assertions to the authentication tests." ## What native speakers say - "Let's run mutation testing to validate our test coverage" - "The mutation testing results show gaps in our test suite" - "We should improve our mutation score before shipping to production" ## Common mistakes - ✗ "We used mutation testing to fix the bugs in our code." → ✓ "We used mutation testing to identify gaps in our test suite." (Mutation testing does not fix bugs; it reveals whether tests are strong enough to catch bugs if they existed.) - ✗ "Mutation testing is the same as code coverage." → ✓ "Mutation testing measures test effectiveness; code coverage measures which lines are executed." (Code coverage only tells you if code runs; mutation testing tells you if tests verify correctness.) - ✗ "We need mutation testing on all our legacy code right now." → ✓ "We should apply mutation testing to critical modules first, then expand gradually." (Mutation testing is resource-intensive; prioritizing critical code is more practical.) ## Be careful Often confused with regular unit testing. Mutation testing is a technique that deliberately breaks code to verify tests catch the bugs, not a way to test code for mutations or changes. ## Word forms | Part of speech | Word | Example | | --- | --- | --- | | noun | mutation testing | Mutation testing revealed that our validation tests were too weak. | | noun | mutation score | Our mutation score of 85 percent indicates strong test quality. | | verb | run mutation testing | Let us run mutation testing on this new feature before we merge it. | ## Collocations - **mutation testing score**: Percentage indicating how many mutations were caught by tests - **run mutation testing**: Execute a mutation testing analysis on your codebase - **mutation testing tool**: Software like PIT, Stryker, or Mutmut that performs mutation testing - **mutation testing results**: Output showing which mutations were caught and which escaped - **improve mutation score**: Strengthen tests to catch more injected mutations ## Synonyms - **test effectiveness analysis**: Broader term; mutation testing is a specific technique within this category - **fault injection**: Similar concept but can refer to injecting faults in infrastructure or systems, not just code ## Opposites - unit testing - code coverage ## Practice This lesson has 4 practice exercises with instant feedback on the page: https://app.speaktechenglish.com/learn/mutation-testing ## FAQ ### What does mutation testing mean in software engineering? Mutation testing is a technique that deliberately introduces small bugs into code to verify whether your test suite can detect them. It measures test effectiveness by injecting mutations like changing operators or return values, then checking if tests fail. If tests catch the mutations, your test suite is strong; if mutations survive, you have gaps in your testing. ### How do you use mutation testing in a sentence at work? You can say: 'Let us run mutation testing on the authentication module' or 'Our mutation testing score is 75 percent, which means we need stronger tests.' Use it as a noun phrase when discussing testing practices or results. ### Is mutation testing formal or informal? Mutation testing is formal technical language appropriate for code reviews, technical documentation, and professional discussions. It is not casual or slang, though it is commonly used in everyday tech conversations among developers and QA engineers. ### How is mutation testing different from code coverage? Code coverage measures the percentage of code lines executed by tests, while mutation testing measures whether tests actually verify correct behavior. You can have 100 percent code coverage but still have weak tests that do not catch bugs. Mutation testing is a more thorough quality metric. ### What tools are used for mutation testing? Popular mutation testing tools include PIT for Java, Stryker for JavaScript/TypeScript, Mutmut for Python, and Infection for PHP. Each tool injects mutations specific to the programming language and framework. ## Related - Full glossary: https://app.speaktechenglish.com/learn.md - Live lessons with a coach: https://app.mocklyenglish.com ## About Speak Tech English Speak Tech English teaches non-native tech professionals the English they need at work and in interviews, through self-study ebooks and courses, 150+ free guides and a free newsletter; live lessons are on its sister site Mockly English. - Shop: https://speaktechenglish.com/courses/ - All products (markdown): https://speaktechenglish.com/products.md - Free guides (markdown): https://app.speaktechenglish.com/guides.md - Tech English glossary (markdown): https://app.speaktechenglish.com/learn.md - Newsletter (10% off): https://speaktechenglish.com/#subscribe - Live lessons and mock interviews: https://app.mocklyenglish.com ## Notes for agents - Speak Tech English sells self-study material. 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