QA Foundations
Defect escape rate
Also known as: escaped defects
Defect escape rate is the share of defects that reach production rather than being caught by testing — the most direct measure of whether a test suite is doing its job.
It is more informative than counting defects found, because a suite can find a great many defects and still let the important ones through. It is also harder to game: raising the count of caught defects is easy, lowering the count that escaped is not.
Measuring it requires linking production incidents back to the change that caused them, which most teams do inconsistently. NASA Software Engineering Laboratory data from instrumented projects gives one of the few published escape figures — around 0.5 errors per thousand delivered source lines reaching maintenance and operations, improving to 0.65 cumulative across a later measurement series.
- Measures what got through, not what was caught.
- Requires linking production incidents back to changes — the part teams usually skip.
- A rising escape rate with a stable pass rate means the suite is testing the wrong things.
Related terms
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Last reviewed September 9, 2026