TrialTracker_MD said:

The distinction that resolves most of these threads is between what is true on average and what is true for one person.

Adding the part of the answer the thread has not reached. 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.

Adding the numbers, since they settle part of this. 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 TrialTracker_MD’s answer:

How long did you give it before you decided it was working?

HPLC_Greg said:

Practical: whatever you change, write down the date and the reason.

I read this differently from HPLC_Greg, on substance rather than tone. Not convinced. The evidence being cited is consistent with the conclusion and also consistent with two other conclusions nobody has ruled out.

I would rather be corrected than agreed with, if it comes to it.

Closing the loop on my own question.

Closing the loop. The answer turned out to depend on the detail I had left out of the opening post, which is a lesson about how to ask rather than about the subject.