I was recently asked to look at a project that had around 60% code coverage and was asked to give my recommendations on what area to focus on to increase the code coverage. There were a lot of unit tests and actually there was around 10% more unit test code than production code so I was a little surprised that the coverage was as low as 60%. Obviously something must be wrong with the tests. A closer look showed that the code that implemented the logic of the application had very high coverage in general and that only the code dealing with external dependencies had low coverage. Also there wasn't that much code so the boilerplate code just to initialize the application was around 15% of the code. The code dealing with external dependencies and that low coverage was slightly less than 10%. So all in all 60 of the possible 75% was covered. Relatively speaking that is 80% - a number I've often heard managers be very happy with (but remember code coverage itself is a shady property to measure).
So what was missing? Well there were a few cases where a few small classes completely were missing unit tests so they would be easy to fix. There were also a bunch of classes that had unit tests exclusively for the happy path of execution and those can be fixed easily too. But I still think it would be possible to bring coverage up above 90%. How is that possible you might think? Well, the boiler plate code had some, but more important the code dealing with external dependencies had a lot of room for improvement. They were much more than simple pass through objects needing no testing and with a simple refactoring like extracting the external call as a protected virtual method of the class you could easily add unit tests for a huge portion of that code to I think.
So my conclusion? The original coverage was not bad. It was covering the important pieces and functional tests (not used when I analysed code coverage) covered a big portion of the code not covered by unit tests. So the situation was not bad. But when you use a code coverage report the right way and look at what and why it is not covered you can always find areas of improvement.