But do they establish a causal arrow? If you're in chronic pain because you're dying of something you're not exactly going to be likely to do a lot of cardio workout. :)
Without care you might conclude, for example, that rock climbing prevents becoming a quadriplegic since there aren't any quadriplegic rock climbers... when in fact to the extent that they're related at all rock climbing sometimes causes it.
Without care you might conclude, for example, that rock climbing prevents becoming a quadriplegic since there aren't any quadriplegic rock climbers... when in fact to the extent that they're related at all rock climbing sometimes causes it.