In some cases, the pre-debate poll was conducted months before the debate. But even if we only focus on the polls that released results at some point in September, the top line is similar: Biden’s gain is 1.4 percentage points, on average. And for what it’s worth, a swing of 1 or 2 points toward Biden is consistent with the effects that first debates have had for challengers in the past — although we should note that sometimes a debate bounce fades after a couple of weeks.

These averages, of course, conceal a lot of variation within individual polls; depending on which poll you look at, Biden’s national lead is as large as 14 points (in the most recent NBC News/Wall Street Journal poll) or as small as 2 points (in a John Zogby Strategies poll). I’ll admit to finding the NBC/WSJ poll intriguing, as it’s the only live-caller national poll we have since the debate, and NBC/WSJ polls tend to stick pretty close to our polling averages. Still, averaging the polls is the right strategy in the long run…

It’s harder to tell, though, what effect the president’s COVID-19 diagnosis may have had on the race. YouGov conducted polls right before and after the news and found Biden ahead by 8 points in both. Morning Consult shows Biden having gained 1 point over the past few days. And Ipsos had Biden ahead by 10 points in a poll conducted after the news of Trump’s diagnosis broke, compared to Biden’s 9-point advantage in a poll conducted after the debate but before the president’s diagnosis. The John Zogby Strategies poll that showed Biden ahead by only 2 points nationally was also conducted after the COVID-19 news, although that’s not a pollster I’d give a lot of credence to.