Generally speaking you're absolutely right, but with diet the social pressures are significant enough to compromise analysis even if the group is paying for it.

Generally speaking you're absolutely right, but with diet the social pressures are significant enough to compromise analysis even if the group is paying for it. There have been quite a few studies on the topic, e.g.: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.1031.9343&rep=rep1&type=pdf http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.103...