Defect Leakage
Defect leakage measures the percentage of defects that escape one testing phase and are only caught in a later phase — found in production instead of QA, or found in UAT instead of system testing — indicating where the testing process itself has gaps worth investigating.
A defect "leaking" from system testing into UAT, for instance, suggests system testing's coverage had a gap in that specific area — tracking leakage by phase over time helps identify whether a particular type of defect (say, integration issues) is consistently slipping past a specific testing stage that should be catching it.
High leakage into production specifically is the most costly and visible version — it's essentially the same underlying metric as escaped defects, viewed through the lens of "which internal phase should have caught this but didn't," useful for retrospectives on process gaps.
