Whenever I join a project, I spend the first few months working at half speed or slower because I investigate every dependency before I make a fix or build a ne

Whenever I join a project, I spend the first few months working at half speed or slower because I investigate every dependency before I make a fix or build a new feature. Even working on an unfamiliar part of a big system slows me down as I ask questions and double-check dependencies. Carelessly changing things you don't understand and expecting the tests to save you does not count as "productivity." Good design decreases the amount you need to know to safely and productively hack on a system. Tests don't. Tests can save an experienced and knowledgeable person from their inevitable mistakes -- they can tell you something is wrong -- but they can't direct you toward the right solution. Someone who doesn't know what they're doing can hack by trial and error until the tests pass, but at that point their code is still likely to be wrong.