I read you go with the smallest part above your hips but below your rib cage. And that smallest part has fluctuated for me. Before it was right at my belly button (at 250ish) and now (at 212) its right below my rib cage. For measurements on "inches lose" I just stick with my belly button for consistency. But there isn't a huge difference between my smallest part of my waist and my belly button, maybe .5inch?
If its measurements given for clothing (which is what I am guessing it was for your study/statistical info) its probably taken at the natural waist (smallest area). Total guess on my part though.
So, what were the numbers?