Regression testing is done to verify that modified code does not break existing tested functionality, and works within the requirements of the system. There are two strategies to regression testing, 1) to run all tests and 2) run a subset of tests based on a test case prioritization technique. Running all of the tests provides the best feedback to determine if new or modified code causes errors within the application, but running all of the tests can be time consuming and cost prohibitive. Running a subset of tests based on a test case prioritization technique can help realize coverage quickly, test features that will be used the most, and reveal possible faults in important features early. There are several different regression testing techniques that look at subsets of test cases in order to determine the best tests to choose in order to achieve the most coverage and discover the most faults quickly.