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What to make of differing post-debate polls (Hint: It's good news for Kamala Harris)

First things first: The theme song of the week is the closing theme from The Sally Jessy Raphael Show.

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Analysis by Harry Enten
CNN — First things first: The theme song of the week is the closing theme from The Sally Jessy Raphael Show.

Poll of the week: A new ABC News/Washington Post national poll taken after last week's debates shows former Vice President Joe Biden leading the Democratic field with 30%. He's followed by Vermont Sen. Bernie Sanders at 19%, California Sen. Kamala Harris at 13%, Massachusetts Sen. Elizabeth Warren at 12% and South Bend, Indiana Mayor Pete Buttigieg at 4%.

This poll seemingly differs significantly with CNN and Quinnipiac University's most recent polls, which have Biden down at 22%, Harris in second at 17% and 20% respectively, Warren in third at 15% and 14% respectively, Sanders in fourth at 14% and 13% respectively and Buttigieg at 4%.

What's the point: Polling can sometimes make your head spin. Depending on which poll you looked at post-debate, you find Biden and Harris within the margin of error of each other or Biden more than doubling Harris' support. Likewise, you could see Sanders in a rather clear second place or in fourth place.

The simplest and best thing to do when polls differ like this is to take an average. Sometimes one of the polls will be wrong and one will be right, but most often the truth is in the middle.

When we average the three post-debate polls, we get this: Biden 25%, Harris 17%, Sanders 15%, Warren 14% and Buttigieg 4%.

Even without account for additional sources of potential error, all the individual polls (ABC/Washington Post, CNN and Quinnipiac) have sampling or margin of errors (+/- 5 or 6 points) that are wide enough to account for the differences they share with the average. Indeed, it's actually a good thing the polls differ. The variation shows pollsters are not herding their results to the average and are producing something close to a random sample of the population. Not herding makes for a more accurate average.

By averaging the post-debate polls, we get a pretty good idea of the effect of the debate. Compare the numbers above with an average of all non-partisan polls I took from two prior to the debate: Biden 30%, Sanders 17%, Warren 14%, Harris 7% and Buttigieg 7%.

Harris debate performance clearly had an effect. She saw a big 10 point jump after winning near universal praise. Meanwhile, Biden is down 5 points (following a panned performance) and Buttigieg and Sanders are down a less significant 3 and 2 points respectively. Warren held steady at 14%.

All told, it's probably safest to say that Biden is still out in front nationally. His lead though has been sliced in half.

Following Biden, it's essentially a three-way race between Harris, Sanders and Warren for second place.

Buttigieg, though, should no longer be considered part of anything close to the top tier. His polling is closer to that of candidates like New Jersey Sen. Cory Booker (2%) and former Texas Rep. Beto O'Rourke (2%).

Of course, the big question is whether these effects actually sustain themselves. Just four years ago, Republican Carly Fiorina experienced a large bump after she took on fellow Republican Donald Trump. Trump, like Biden, watched his polling decline.

That cycle, the debate effects turned out to be only momentary.

Maybe this year, it will be different.

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