Every cold shower study is fundamentally flawed as it can only be done on healthy individuals. This skews the result. The subconscious selection is often done before the official selection, this makes it really hard to do right.
Could you explain what you mean by that? Why can't unhealthy individuals participate? And even if they can't, are study results still not applicable for healthy people?
You could also argue that almost all studies are done with adults, or individuals who aren't deceased, etc.
PS: Look up the Doomsday Argument
A core reason why we need to do as many trials as possible for different interventions is that there's no one-size-fit all approach. I believe that some things will work for some and not others.
The bottleneck is in having a standard framework to validate the impact of interventions and understanding "whether or not it works for you" with empirical evidence (not anecdotes)
essentially, a pre-staged shillbot with no history. 4/10 effort, try harder, get some fake repost history in there first