Frankly, I am not sure what this means. Just because a study is not perfect does not mean we cannot learn from it. So we cannot be sure that the well-being of the patient's in the study improved due to the upper cervical work done. Funny thing though, when I was in school, the medics wanted to attribute all improvement that patients had under chiropractic care as being due to placebo effect. They don't say that anymore, they have taken up manipulation.
Chiropractors say it now.
Oh yeah, there are none. Of course, I don't know if that is true. I do know that if make your clinical decisions based only on controlled, blinded studies, you will not be practicing.