Dr.LipidDallas said:

The honest answer is that the effect is real, the magnitude is contested, and the individual variation is larger than either.

Adding a me-too, because a thread of one person's experience is not much use. I had assumed I was the exception until I read this.

lisa_labSD 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…

Correct as far as it goes. The part it does not cover is what to do when the honest answer is "not enough data", which is more often than anyone likes.

lisa_labSD 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…

Bookmarking. The distinction being drawn above is the one nobody else makes. Taking it to my next appointment.

From the other side of the consultation, briefly. Take it one variable at a time. Almost every unanswerable question in these threads is unanswerable because three things changed in the same fortnight, and no amount of subsequent argument can untangle them after the fact.

Moderator note: good thread. Keeping it here rather than moving it, because the question is general enough to be useful.