Programming, Work

Code Coverage 90% meaningless

I have always been quite sceptical about the value of code coverage metrics. Most of the teams I have seen who have been productive and produced code with few defects in production have not used code coverage as a metric. Where code coverage tends to be obsessively tracked it seems to be more as a management reporting metric (often linked to “Quality”) and rarely seems to correlate with lower defects or malleable software instead it often appears in low-collaboration or low trust environments.

Code coverage has most benefit in an immature unit testing environment or in a “test-after” team. With test-after you have to have code coverage to ensure that people remembered to test all the possible execution paths. My personal preference is to push TDD as a practice in preference to code coverage because a side-effect is that you get 100% code coverage.

Code coverage is also quite a different beast to static or complexity analysis of code bases. Static analysis is a useful tool and some complexity measures actually make good indicators of the “quality” of the code base. It is also not the same as instrumented code, which is invaluable with dealing with code you’ve inherited or to discover how much of the codebase actually gets used in production.


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