Structural coverage criteria are often used as an indicator of the thoroughness of testing, but complete satisfaction of a criterion is seldom achieved. When a software product is released with less than 100% coverage, testers are explicitly or implicitly assuming that executions satisfying the remaining test obligations (the residue) are either infeasible or occur so rarely that they have n e gligible impact on quality. Violation of this assumption indicates shortcomings in the testing

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... Monitoring in the deployed environment, even in the beta test phase, is typically limited to error and sanity checks. Monitoring the residue of test coverage in actual use can provide additional useful information, but it is unlikely to be accepted by users unless its performance impact is very small. Experience with a prototype tool for residual test coverage monitoring of Java programs suggests that, at least for statement c o verage, the simple strategy of removing all probes except those corresponding to the residue of coverage testing reduces execution overhead to acceptably low levels.