Test Case Effectiveness
Test case effectiveness measures how well a test suite actually catches real defects — commonly evaluated through mutation testing (do the tests catch deliberately introduced bugs) or by tracking how many escaped defects existing tests failed to catch despite covering the affected area.
A test suite can execute successfully and still be largely ineffective if its assertions are too weak to actually catch wrong behavior — effectiveness asks the sharper question coverage percentage alone can't answer: not just "did a test run," but "would that test have actually failed if the code were broken."
Tracking effectiveness over time — reviewing why a test suite missed a specific escaped defect despite nominally covering that code — is one of the more valuable, if under-practiced, testing retrospective activities, directly informing which existing tests need stronger assertions rather than just more tests being added elsewhere.
