[sohex]

I don't think you can draw meaningful conclusions from a self-enforced system like Weight Watchers either though. Self reporting particularly when it comes to anything diet related is going to be heavily skewed. There will always be social pressures within a group like that to over report exercise and under report intake.

[falcolas]

If you can't trust the results of a self-selected group of individuals who are paying a good chunk of cash for the express purpose of loosing weight... who the hell can you trust?

[sohex]

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=10.1.1.103...