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(CNN) —  

First things first: The theme song of the week is from the show Report to Murphy starring Michael Keaton.

Poll of the week: A new CNN national poll of the 2020 Democratic presidential primary gives Joe Biden 28%, Bernie Sanders 20%, Kamala Harris 12%, Beto O’Rourke at 11% and Elizabeth Warren 6%

Other polling confirms that Biden is ahead of Sanders and both of them are well ahead of the pack.

What’s the point: If you go on social media platforms like Twitter (a second home for many journalists), you’d be shocked to learn that Biden was the frontrunner in the polling so far. Fans of other candidates (especially Sanders) make their presence known quite loudly and can leave you left wondering if the polling is missing something.

My advice: Be very, very careful in looking at Twitter or other social media to understand who is ahead or behind in the Democratic race. The dividing lines in the Democratic primary are such that they expose social media’s weaknesses in accurately capturing the sentiment of the Democratic electorate.

Age is one of the biggest indicators in whom Democrats are choosing in the primary right now. Our CNN poll finds, for instance, that Sanders leads Biden by a 32% to 19% margin among those younger than 45. The same poll showed Biden with 36% and Sanders in third with 8% among those 45 years or older. (CNN’s Iowa poll revealed a similarly large age divide.)

The problem is that social media skews young. I examined three Pew Research Center polls from 2016 and 2018 and looked at Democrats who said they used Twitter. On average, they were 18 points more likely to say they were under the age of 50 than the overall population who said they were Democrats in the same polling. That’s a large enough difference to account for all of Biden’s lead in CNN’s polling.

The age gap also helps to partially explain why social media seemed very skewed toward Sanders in the 2016 primary. That year, Sanders won the younger-than-45 vote by 26 points in the average state exit poll. Hillary Clinton, on the other hand, won those 45 years old and older by 32 points. And because those 45 years old and older made up about 60% of the Democratic primary electorate, she won the nomination handily over Sanders.

In other words, Twitter basically takes the 3:2 advantage Democrats 45-years-old and older have over those under the age of 45 in the makeup of Democratic primary voters and reverses it to make a 3:2 advantage for Democrats younger than 45.

Age is not the only reason why I would be wary of trusting social media to understand the Democratic primary. It’s about ideology, too. Although CNN’s latest national poll didn’t reveal too much of an ideological gap in 2020 primary choices, other polling certainly has. CNN’s Iowa poll put Sanders up by 9 among liberal likely caucusgoers, while Biden was ahead by 18 points among those who identified as being more moderate. A Monmouth University national poll revealed a similar ideological gap.

Liberal Democrats are especially “loud” on social media. According to a 2018 Pew Research Center study, liberal Democrats on social media are about 15 points more likely to encourage others to take action on issues that are important to them than moderate or conservative Democrats.

(That would certainly explain why my Twitter mentions seem to skew toward liberal Democrats compared to what polling indicates is a much more even split between liberal and moderate or conservative Democrats in the party at-large.)

Indeed, when the Pew Research Center asked about five different political support activities, from taking part in a group that shares an interest in a cause or an issue to using hashtags to support a cause, the ideological effect held. Liberal Democrats were about 15 points more likely to do at least one of these than moderate or conservative Democrats.

When you combine age and ideology, the gap grows even wider. That is, liberal Democrats under 50 are by far the most likely to use social media in order to support their candidate of choice.

We don’t know if the current age and ideological gaps will hold through the rest of the primary season. That is, we don’t know that Biden (Sanders) will continue to look weaker (stronger) on Twitter than in the polls.

What we do know is that Democratic primary candidates who receive a disproportionate amount of their support from liberal and/or younger voters will have a larger percentage of the support of those on social media than they will in real life.