Ruby's contemporary test coverage tools all lie, exaggerating coverage through false-positives and creating a false sense of security; minitest-coverage tries to address this. Coverage Analysis Tools rely on tracing facilities built into ruby’s VM. You run your tests, and collect data. Seems simple, but that’s a very flawed approach that buffers your coverage numbers up falsely. I’ve witnessed false coverage by as much as 60%, but it could be even worse. Worse, the tracing facilities currently make it impossible to get truly accurate numbers. Even so, they can be improved to be much more accurate.