I hate the way this saying is commonly used today. I think a literal interpretation is untrue, but many people feel that a literal interpretation is true. For instance, humans get better at speaking whatever language they are surrounded with, even though that is not being actively measured by some metric. It is probably being measured by some implicit cycle in the human brain, but that is not the kind of explicit measurement that people would demand based on this saying. Some other examples that you can improve without an explicit measurement:
- Camera stability (you can usually "see" it immediately, without an engineering metric)
- Large changes in customer satisfaction (for fine tuning you probably need a metric, but for large changes, it will be obvious)
- Kindness
I'm not saying that measurement is purposeless. Just that it is not always necessary. Not everything needs to be measured. Also, why do generally smart people buy this platitude, when others will obviously not be taken as law? I don't see engineering orgs living by "closed mouths don't get fed", or "tidy desk, tidy mind", or "if momma ain't happy ain't nobody happy". But somehow "You can't improve what you don't measure" became law.
I realize this is only tangential. I guess I've been saving this rant for a while.