The case for a big Comey effect hinges on the large decline in Mrs. Clinton’s lead in the polls that followed the letter. In the FiveThirtyEight model, for instance, Mrs. Clinton had nearly a six-point lead heading into the Comey letter, but just a three-point lead one week later: an apparent three-percentage-point shift against Mrs. Clinton. It seems reasonable, as many have argued, to attribute much of that decline to Mr. Comey’s decision.

But the Upshot/Siena poll of Florida is one of several surveys that challenge this interpretation. That poll was completed the night before the Comey letter, but it was not released until Sunday, two days later — a longer lag than usual, since Sunday is seen as a better day for news media coverage than Saturday.

Some analysts have used poll aggregators or forecasting models to measure the effect of the Comey letter, and they have implicitly treated this Upshot poll, and others conducted before the news but released after, as evidence of a Comey effect. But it can’t be; for example, none of the people we polled for our survey knew about the letter.

Unfortunately, there is not much polling from this narrow period before the Comey letter and well after the third presidential debate. But it was accepted at the time that Mrs. Clinton’s lead was slipping heading into the morning of Oct. 28. The ABC/Washington Post tracking poll conducted over the same period as the Upshot/Siena poll of Florida, for instance, showed Mrs. Clinton’s lead at just two points, down from a double-digit lead after the third debate. That poll was also released after Mr. Comey’s letter.