Clear: Everybody knows what you mean. You know what you mean. Observable: What do you see when you see more of it? This doesn’t mean you will necessarily have already observed it but it is at least possible to observe and you will know it when you see it. Useful: It has to matter to some decision. What would you do differently if you knew this? Many things we choose to measure in security seem to have no bearing on the decision we actually need to make.
