Test factoring: focusing test suites for the task at hand

Frequent execution of a test suite during software maintenance can catch regression errors early, indicate whether progress is being made, and improve productivity. However, if the test suite takes a long time to produce feedback, the developer is slowed down, and the benefit of frequent testing is reduced. After a program is edited, ideally, only changed code would be tested. Any time spent executing previously tested, unchanged parts of the code is wasted. For a large test suite containing many small unit tests, test selection and prioritization can be effective. Test selection runs only those tests that are possibly affected by the most recent change, and test prioritization can run first the tests that are most likely to reveal a recently-introduced error.