Another issue with the subjective experience of fatigue is that many of us can manage fatigue, to a degree, with symptom management techniques i.e. by pacing and resting etc. So, for example, if I rest for extended periods then my fatigue/exhaustion improves, but if I push against my symptoms then my experience of fatigue/exhaustion worsens. i.e. my fatigue can worsen when I increase my activity levels, and my fatigue can improve when I decrease my activity levels. So an improvement in fatigue can actually indicate worse incapacity. So that's another reason why fatigue is a crap measure of the overall illness. My own preference is to have reduced symptoms by reducing my activity; that's how I choose to manage, and live with, my illness. But if my symptoms improve and I'm less active then my overall health has not improved; it's just a better subjective experience for me but with worse incapacity. I think most of us would agree that a measure of overall physical function is a better indicator of changes in the underlying illness, than a snapshot measure of subjective fatigue. Although, physical function can also be misleading if the increased activity is at the expense of symptoms and quality of life.