There is something I call the 10% improvement problem. Basically, it's hard to track a change of a couple tens of percent in most abilities / states of being.
In some cases, there is a definition problem. What does it mean for your to be "twice" as happy as yesterday?
Assuming you do know, would you notice a 30% change in happiness? A 10% change?
Can you tell if you're being twice or half as productive? Maybe if you do the same thing everyday. But when faced with a new task, it's hard to tell whether the task is inherently difficult or if you're being uncharacteristically unproductive.
Even things that I feel should be eminently measurable, aren't. The effectiveness of a training program in building muscle, for instance. Heck, it's even hard to precisely measure muscle mass at 10% accuracy.
And that is saying nothing about confounding factors. You don't live a regimented life in a sterile box. Any comparison is necessarily going to be pears to apples.
And yet, 10% changes matter. Stack 10 of them and you've doubled whatever you were trying to improve. The problem is, how do you know what to stack?
Over a long period of time, these incremental changes are definitely noticeable. If you're on average 30% more effective every day, that will be noteworthy at the end of the year.
But you don't have the time to wait long periods for every single change you want to make. So you stack many of them, see if things improve on the long run, and accept that part of what you're doing may be pure cargo cult self-improvement.
This is an open problem, I have no solution to offer, but any help is appreciated.