anders_CPH said:

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

This is where I part company with the consensus forming above. Filing an objection on the n=1 problem. Personal experience is genuine evidence about one person and very weak evidence about anybody else, and this thread keeps promoting the first into the second.

anders_CPH said:

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

Agreed, and one caution: a single measurement is not a measurement. Anything that moves day to day needs a trend before it means anything at all.

anders_CPH said:

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

Saving this. It is the first explanation that did not require me to already understand it.

Adding the clinical framing, because it changes how the question reads. Order of operations matters more than any single choice here: establish a baseline, change one thing, wait long enough for it to express itself, then measure again under the same conditions.

Dr.KarenChen said:

Order of operations matters more than any single choice here: establish a baseline, change one thing, wait long enough for it to express itself, then…

Same pattern here, and in the same order.