Prisons give you the control needed, but prisons generally are not realistic to how people could live their lives. When you are locked in a cell most of the day that limits movement (in ways different from an office where people get up to go to meetings and the like). Prisons will get you your 20 minutes a day of exercise, but it isn't representative of how most people will exercise (even counting only those who go to the gym). As such you can get a lot of data but it is unknown which data applies to normal people who live lives in ways that are likely different in ways that matter.

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