Defect leakage

Defect leakage refers to the defect Found \ reproduced by the Client or User, which the tester was unable to found.
Defect leakage is the number of bugs that are found in the field that were not found internally. There are a few ways to express this:
* total number of leaked defects (a simple count)
* defects per customer: number of leaked defects divided by number of customers running that release
* % found in the field: number of leaked defects divided by number of total defects found in that release
In theory, this can be measured at any stage - number of defects leaked from dev into QA, number leaked from QA into beta certification, etc. I've mostly
used it for customers in the field, though.