The 25% (comment, analysis and ramblings)
There's a group in the UK specifically for the severely-affected (I used to be a member, when I was, and it was a huge help). It's called the 25% Group, the name taken for the proportion of patients either bedbound or housebound. I'd never been able to find peer-reviewed evidence for the 25% figure*, though it seems to be widely accepted and I have no good reason to challenge it. Peer-reviewed, published support for the figure would give it more weight eg in advocacy work - and 25% is a shocking figure.
[* this paper cites a 1990 book co-authored by Dr David Bell, and a 2001 Action for ME membership survey, that I can't access]