A narrower follow-up, since the general answer is now clear:

What would you measure differently if you were starting again?

kate.chem said:

For anyone reading later: the numbers in this thread are worth checking against a primary source before you act on them, including mine.

Pushing back on kate.chem here. Pushing back on the consensus forming here. Consistency is not the same as correctness, and a board that agrees with itself quickly is usually agreeing with the first person who sounded certain.

kate.chem said:

For anyone reading later: the numbers in this thread are worth checking against a primary source before you act on them, including mine.

Agreed, and the size of the effect matters as much as its existence. Something real and small gets treated here as though it were real and decisive.

kate.chem said:

For anyone reading later: the numbers in this thread are worth checking against a primary source before you act on them, including mine.

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

OP back with an update, since a thread like this is useless without one.

Update, and it is the boring outcome: nothing was wrong. The measurement I was reading had moved less than its own week-to-week variation, which somebody said upthread and I did not believe.