Clinical perspective, offered as context rather than as advice. The useful move here is to separate what is established from what is widely repeated. Those two sets overlap less than the confident tone of most write-ups suggests, and the second set is where nearly all the disagreement on this board comes from.

Dr.PeteFamMed said:

The boring version of this is the one that works, and the boring version is: measure a baseline, change one variable, wait, measure again under the…

Coming at Dr.PeteFamMed’s question from a different direction. The honest answer is that the effect is real, the magnitude is contested, and the individual variation is larger than either. Those three things can all be true at once, and most arguments here are two people holding different parts of that.

One concrete data point for the thread. Practical: whatever you change, write down the date and the reason. In three months the reason is what you will have forgotten, and the reason is what makes the record worth having.

One thing that is still open after Dr.PeteFamMed’s answer:

What would you measure differently if you were starting again?

Moderator note: a couple of off-topic posts removed. Thread quality here is what the rules are for. Keep it up.