On the XWiki project, we've been pursuing a strategy of failing our Maven build automatically whenever the test coverage of each Maven module is below a threshold indicated in the pom.xml of that module. We're using Jacoco to measure this local coverage.
We've been doing this for over 6 years now and we've been generally happy about it. This has allowed us to raise the global test coverage of XWiki by a few percent every year.
More recently, I joined the STAMP European Research Project and one our KPIs is the global coverage, so I got curious and wanted to look at precisely how much we're winning every year.
I realized that, even though we've been generally increasing our global coverage (computed using Clover), there are times when we actually reduce it or increase very little, even though at the local level all modules increase their local coverage...