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Mutant states and metrics

This page should shed some light on the different outcomes a mutant can have and the different metrics you'll find in any given mutation testing report.

Are you new to mutation testing, see what is mutation testing?.

Mutant states

A mutant can have one of the following states:

  • Pending The mutant has been generated, but not run yet. This is a temporary state and will be changed once the mutant has been tested.
  • Killed When at least one test failed while this mutant was active, the mutant is killed. This is what you want, good job!
  • Survived When all tests passed while this mutant was active, the mutant survived. You're missing a test for it.
  • No coverage The mutant isn't covered by one of your tests and survived as a result.
  • Timeout The running of tests with this mutant active resulted in a timeout. For example, the mutant resulted in an infinite loop in your code. Don't spend too much attention to this mutant. It is counted as "detected". The logic here is that if this mutant were to be injected in your code, your CI build would detect it because the tests will never complete.
  • Runtime error The running of the tests resulted in an error (rather than a failed test). This can happen when the test runner fails. For example, when a test runner throws an OutOfMemoryError or for dynamic languages where the mutant resulted in unparsable code. Don't spend too much attention looking at this mutant. It is not represented in your mutation score.
  • Compile error The mutant caused a compile error. This can happen in compiled languages. Don't spend too much attention looking at this mutant. It is not represented in your mutation score.
  • Ignored The mutant wasn't tested because it is ignored. Either by user action, or for another reason. This will not count against your mutation score but will show up in reports.


Based on these states, we can calculate the following metrics:

  • Detected killed + timeout
    The number of mutants detected by your tests.
  • Undetected survived + no coverage
    The number of mutants that are not detected by your tests.
  • Covered detected + survived
    The number of mutants that your tests produce code coverage for.
  • Valid detected + undetected
    The number of valid mutants. They didn't result in a compile error or runtime error.
  • Invalid runtime errors + compile errors
    The number of invalid mutants. They couldn't be tested because they produce either a compile error or a runtime error.
  • Total mutants valid + invalid + ignored + pending
    All mutants.
  • Mutation score detected / valid * 100
    The total percentage of mutants that were detected. The higher, the better!
  • Mutation score based on covered code detected / covered * 100
    The total percentage of mutants that were detected based on the code coverage results.

Test states and metrics

A test can also have state with regards to mutation testing.

  • Killing The test is killing at least one mutant. This is what you want.
  • Covering The test is covering mutants, but not killing any of them. The coverage information should be available per test to provide this test state.
  • Not covering The test is not even covering any mutants (and thus not killing any of them).
  • Total not covering + covering + killing Total number of tests.