It would seem to me that there is plenty of potential that managing one's diabetes better could lead to better glycaemic control/lowering of HbA1c in some people which could improve scores on average. For example, I have a distance relative, a widower, who prays every night he won't wake up - this was even before he had many complications from the diabetes. He has bags of sweets in his house and generally doesn't look after himself as a diabetic should. I'm no expert on diabetes but some or all of these would seem to be true: (i) diet can play a big part in controlling it. There is some foods (and drinks) one should generally avoid or only take in very small quantities. If a person wasn't doing this well, a motivational program (which Simon Wessely said was involved in intervention) would likely improve HbA1c levels. (ii) more exercise can stabilise blood sugar levels as far as I know (at least in type 2 but I think I've seen also in type 1). There would be plenty of people with diabetes who for one reason or another haven't prioritised this/aren't doing enough/could do more exercise. (iii) measuring blood sugar levels - some people may not do this sufficiently often to calculate when to eat/how much to eat and when to take insulin. Related to this point is whether people take their insulin at the optimum times. (iv) adherence to other drugs that might help with blood sugar control. So to me, I can see how a management program or if necessary a CBT program if somebody wasn't complying well enough, could on average improve scores in diabetes. Such a program could be indirect also. For example, taking the example of my depressed distant relative: if his depression was treated, he might be more motivated to follow diabetes advice. In ME/CFS, I think it is much less clear cut what we should be recommending people. In particular, I'm not convinced about the model underlying GET and GET-based CBT. So just because a CBT intervention might help on average in diabetes, doesn't tell us that much about what a GET-based CBT intervention (for example) will do in ME/CFS.