Comment by throwaway74432

You're right, it isn't complicated. The beauty of a true good measure (ie, a scale or other measuring device) is that you can track actual discrete progress over a very short time span. If you didn't know your hair grew, could you track the amount that it grows in 24 hours by looking in a mirror? With a measuring tool, you can. That's a good measure. Compared to it, looking in the mirror is a poor measure. That's why scientists use measuring devices when trying to track quantities, instead of "eyeballing" it and claiming its good enough because they noticed some change.