Gilb’s Law: “Anything you need to quantify can be measured in some way that is superior to not measuring it at all.” Gilb’s Law doesn’t promise you that measurement will be free or even cheap, and it may not be perfect – just better than nothing.
Source: Peopleware – Productive Projects and Teams, Third Edition, Tom DeMarco and Timothy Lister, Dorset House Addison-Wesley, 978-0-321-93411-6