Escaped Defect Rate
Of the bugs that reached users, which ones should the test suite have caught? Escaped defect rate is the slowest and most honest measure of test effectiveness: not “would the tests catch a hypothetical mutant?” but “did they catch the actual failures, judged after the fact?”
Sensor properties
| Property | Value |
|---|---|
| Oracle strength | Medium — classification of “should have caught” is judgment |
| Independence | High — production failures cannot be gamed by the suite |
| Scope | System |
| Feedback latency | Weeks |
| Actionability | Guiding — points at the layers of the suite that leak |
| Type | Retrospective |
What it cannot detect
Defects nobody reported, and defects attributed to the wrong cause. It also lags badly: it tells you about the suite you had, not the suite you have. Pair it with mutation testing for a fast proxy and with incident correlation for the cost side.