A way to make this fundamental statistical error more understandable is to note the following analogy. A study examines two groups of people, one group from the north and one group from the south. The north group was then divided into the tall northerners and the short northerners on the basis of height. The researchers then inappropriately compared the tall north group to the entire undivided south group.
Naturally, the tall north subgroup who were selected on the basis of being tall would on the average be taller than the undivided total south group. (Similarly, the tall north group would also be likely to have a statistically greater value for a foot to waist measurement or waist to head measurement than the average value for the undivided south group.) However, one could not say that because of this comparison, that there is a unique subgroup that exists in the north group of tall people that doesn’t exist in the south group. This is fundamental error that the authors of this study made. It was perhaps less obvious because the measurements they used involved PET scanners and cardiac measurements, but that doesn’t make this inappropriate statistical analysis any less wrong than this example of people divided on the basis of height.