> no possible blind study, Not sure I agree with this. I am definitely not saying this should be done, but as a thought experiment: Imagine you worked for Fi

> no possible blind study, Not sure I agree with this. I am definitely not saying this should be done, but as a thought experiment: Imagine you worked for Fitbit, and could choose to undercount steps slightly. You have a cohort of users who will walk each day until they reach 10k steps. You undercount steps for a random subset so that they end up walking 10.5k. You repeat this experiment so you make the folks with a 5k goal actually walk 5.5k, etc This is a pretty good blinded study. The difference is slight enough that the users don't notice the extra increase in distance consciously. (Obviously you have to take into account the risk of churning people from the service somehow.) If you were to check all cause mortality somehow (look up public death records? Some other way?) That would seem like a pretty good study design. Again, perhaps unethical to do this without consent; but not impossible; you could ask a cohort of users to opt in to this sort of study. If the claim is that it's distant dependent, this would likely take you a lot closer to causality.