I'm not aware of any prevalence studies using the NICE criteria. But in any case, the NICE criteria in themselves may describe little more than chronic fatigue. Though they would seem to require “post-exertional malaise / fatigue,” in fact, they make even “general malaise or ‘flu-like’ symptoms” optional. As a result, they actually require only “a substantial reduction in activity level characterised by...fatigue (typically delayed, for example by at least 24 hours, with slow recovery over several days)" – plus at least one other symptom, possibly insomnia. When I was using GET, I wasn't yet in the severe category. I would likely have stayed out of that category too had I been advised to pace myself rather than to exercise. I had been diagnosed with CFS, and I didn't know enough at the time to request that my treatment be personally tailored. I was so motivated to get well, that I followed my treatment instructions as best I could, despite my repeated relapses. At issue here is which definition(s) will be used to research alternative therapies. According to the ICC authors, "It is imperative that research for ME be carried out on patients who actually have ME:" If only CFS cohorts are used, on the other hand, treatment results can't be assumed to apply to patients with ME.