Mutation testing (or mutation analysis or program mutation) is used to design new software tests and evaluate the quality of existing software tests. Mutation testing involves modifying a program in small ways. Each mutated version is called a mutant and tests detect and reject mutants by causing the behaviour of the original version to differ from the mutant. This is called killing the mutant. Test suites are measured by the percentage of mutants that they kill. New tests can be designed to kill additional mutants. Mutants are based on well-defined mutation operators that either mimic typical programming errors (such as using the wrong operator or variable name) or force the creation of valuable tests (such as dividing each expression by zero). The purpose is to help the tester develop effective tests or locate weaknesses in the test data used for the program or in sections of the code that are seldom or never accessed during execution. Mutation testing is a form of white-box testing. Introduction Most of this article is about "program mutation", in which the program is modified. A more general definition of mutation analysis is using well-defined rules defined on syntactic structures to make systematic changes to software artifacts. Mutation analysis has been applied to other problems, but is usually applied to testing. So mutation testing is defined as using mutation analysis to design new software tests or to evaluate existing software tests. Thus, mutation analysis and testing can be applied to design models, specifications, databases, tests, XML, and other types of software artifacts, although program mutation is the most common. Overview Tests can be created to verify the correctness of the implementation of a given software system, but the creation of tests still poses the question whether the tests are correct and sufficiently cover the requirements that have originated the implementation. (This technological problem is itself an instance of a deeper philosophical problem named "Quis custodiet ipsos custodes?" ["Who will guard the guards?"].) The idea behind mutation testing is that if a mutant is introduced, this normally causes a bug in the program's functionality which the tests should find. This way, the tests are tested. If a mutant is not detected by the test suite, this typically indicates that the test suite is unable to locate the faults represented by the mutant, but it can also indicate that the mutation introduces no faults, that is, the mutation