While I agree that PEM is the key feature of ME rather than the rather amorphous and uninterpretable 'fatigue', there is a huge problem with using frequency and severity of PEM as a measure of severity of illness. Frequency of PEM surely depends on the ability to pace to avoid it, and severity depends on how much you have pushed past your limits and for how long, not on how ill you are. That is, unless you are so ill that even moving or thinking trigger PEM, so you are in a state of permanent PEM. Some sort of measure of muscle and brain fatiguability might be more useful. The 2 day CPET is the obvious one, but is not a good idea for most of us because it triggers PEM that can be long lasting. We need an equivalent test that doesn't trigger PEM. Edit to add: not only fatiguability, but abnormally long recovery time is key, I think.