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.
I'm not sure but we might be entering the world of the Unknown with some of the assumptions. You can equally go into PEM if you have a cold and are sitting on the couch resting, so pacing isn't the only factor? PEM severity and frequency are not just about exercise they are cognitive too and arise from the sum of all activity but also can seem to be quite random in terms of progression (from my own experience and that of others I've read on the forums) so it's not just about muscles. Severity does not necessarily correlate to how much you've pushed ...I think that's a big assumption...it could equally be attributable to other things.
Putting that aside though, inthe broadest sense any treatment should be robust enough to prove that it is effective against all of these factors not just the ones we academically pluck out of the air such as CPET etc.
So as well as having slices of objective measures that may or may not give you a full picture, you also need a measure of the whole success of a treatment. Certainly this is true for new studies where no clarity exists of confirmed links with measures and reality.
THis was my context in my previous post suggesting the measurement of PEM severity and frequency...it tells you whether a treatment has worked or not in the broadest sense.
Epilepsy for instance is a good example in terms of an illness described by periods of relative stability and episodic symptoms. The life of sufferers gets progressively worse the higher the frequency of seizures where in some severe cases seizure frequency is so high that this is very limiting. It is also thought that the more seizures the patient has, the higher the chance of these recurring. This may be the case for PEM, I'm not sure we know that though. However I think the parallel is a reasonable one.
It is customary to subjectively measure the frequency and severity (and type) of seizure as a measure of this condition and to tell how well the patient is responding to treatment. You also have the EEG machine to see what's going on before, after and during a seizure. However there is not a practical way of measuring progress objectively in real life on a constant basis. In fact the EEG machine stays where it is most of the time and the respondent has to visit the hospital and have a seizure induced so they can measure what's going on. So this becomes an intrusive test that you don't want to do too frequently (like the CPET). That leaves the subjective measurements as the more important measure of what progress is being made due to practical reasons.
I'm pretty sure if I had significantly less PEM episodes than I did before I had treatment, I would class that treatment as a success? Objective measurements tend to show you only one thing not the whole. You have to prove that the objective measure is giving you a true reflection of treatment success/disease progression. I'm not sure any of the examples listed so far have a direct corellation or are particularly practical. This means we need to look at the. It picture first and then drill down.
Chalder picked the wrong subjective measure and used rubbish methodology, that doesn't mean the premise of trying to measure the big picture was wrong.
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Which leads me to believe this study is probably carp. 
(if people wonder, it's from moneysavingexpert.com forum where it's used to avoid the bad language filter)
Too much needs doing here for me to read and take in things well... Very crashed today after journey down followed by overdoing things trying to help (in very small ways) OH get new van in live-able state.