[steve_adams_86]

Especially in dietary studies. You either spend a lot on high quality, controlled studies where you can nail down parameters (takes a LOT of labour), or you spend on facilitating much larger studies where you make up for precision and control with volume.

There are trade offs in either case and some types of research where one is more suitable than the other. But the best case is a combination of the two, and it's exceedingly rare.

Maybe there are other options but this seems to be the polar nature of these studies from what I've seen.