The figures, for anyone assembling their own picture. 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.
A narrower follow-up, since the general answer is now clear:
How would you tell the difference between that and the alternative explanation?
bri_stats said:Practical: whatever you change, write down the date and the reason.
Pushing back on bri_stats here. The objection nobody has made: this only works if the measurement is doing what we assume it is doing, and that assumption has not been tested here.
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Shop Reference Standardsbri_stats said:Practical: whatever you change, write down the date and the reason.
That is right for the common case. Whether it is right for the case being asked about depends on a detail that has not been given.
Closing the loop on my own question.
Closing this out. What actually changed my mind was the point about waiting for a trend instead of reading a single measurement, which is embarrassing in hindsight and worth saying anyway.