Thanks for digging all that up.Here's some workplace data on CIS norms: Fatigue among working people: validity of a questionnaire measure, see Table 2
White collar worker 20.3 (10.1) mean (SD)
Blue collar: 21.9 (11.4)
After hernia: 32.9 (11.7)
Pregnant: 33.9 (13.0)
Mental: 39.6 (12.1)
On these figures (which are from some of the FITNET co-authors), 30 does look high for a healthy mean. Though maybe adolescents are more fatigued than adults...
A nature paper on bone marrow transplation gives mean fatigue levels under 30 even though around 1/3 of theseare stated as having severe fatigue.
I've also seen >35 as the threshold for severe fatigue eg a Bleijenberg paper on Rheumatoid Arthritis.
Ah, maybe this is it: Is a Full Recovery Possible after Cognitive Behavioural Therapy for Chronic Fatigue Syndrome?
The figure from the full recovery paper is the one I had in mind.
Also note that none of them had SDs close to 5.