Actually, I think you are on to something here (even though the trial particpants are scoring themselves relative to their pre-illness self).And on a closely linked subject...
It seems to me that a score of 12 or above on the Chalder questionnaire, would indicate a trend to an overall worsening of health...
If, on average, the answers tend towards "worse than usual", which would be the case for an average score 12 or above (a score of 11 being neutral, and not indicating any change either way), then that would indicate an overall worsening of health.
So how can a score of '18' indicate a 'normal range', when it actually indicates an overall deterioration in health?
The original bimodal 'normal range' of '3 or less' did actually indicate an improving participant.
But the new likert 'normal range' of '18 or less' includes participants who are worsening in overall health.
Do you think these interpretations are solid?
The key point is that the threshold of 18 is set on the basis of a general population so that for the general population 18 is supposed to be within the normal range. So, pull someone out of the general population with a score of 18 and they have worsening fatigue on 7 out of 11 items - how is that 'normal'?