2. Not an interval scale
Yes, pretty tough to make an interval scale. I'm pretty sure there is no evidence SF36 is an interval/ratio scale and that's true of most questionnaires generally. Every now and then statisticians complain this invalidates some statistical interpretations, but this view never seems to get much traction
I suspect it is an ordered scale rather than simply giving classes so had some utility. Also, even if some patients disagree to about some of the ordering the main thing measured is within-subject score (pre/post) and so patients presumably score themselves consistently there.
Also, with the SF36 for modest change most item scores don't change at all ie remain 'not limited at all' or 'limited a lot' (which includes 'impossible'). Probably only 2 or 3 ex 10 questions change for most people and in each case they are moving from:
- limited a lot > limited a little, or
- limited a little > not limited
not sure if this helps make things more consistent or not.