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Looking at the poll numbers shown by realclearpolitics.com for 2020 Democratic Presidential Nomination. Specifically Biden's poll numbers. There seems to be two trend-lines for him instead of one. One hovers 29-34 and another distinctly lower between 22-26. To put it another way: since 7/1 he has 18 results in the 29-34 range and interspersed, you'll see the occasional blip going down to 22-26. Since 7/1 there are only 8 results in the 22-26 range. Going back further seems to show the same high/low pattern.

Biden's poll numbers graph 2

Something's happening here. There is a weird high versus low pattern in Biden's poll results; most of the numbers fall very close to either the high or the low with very few in between. There must be a different methodology or some other explanation underlying the lower results as compared to the higher results.

Why does Biden show seemingly two trend-lines with his polls?

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  • I also can't find that graph in your question. Based on your picture, I'd think it's just the sampling error. – JJJ♦ Aug 13 '19 at 19:54
  • Are you trying to ask if there is any consistent difference between the polls that have Biden at ~30% vs the polls that have him at 22%? For example, maybe one has more landlines while the other finds people on the internet? – divibisan Aug 13 '19 at 19:54
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    @divibisan The issue is not with their website but rather I'm trying to understand the reasons for the roughly 8 point difference between the highs and the lows. Yes I'm looking for the consistent diffence. – D_Bester Aug 13 '19 at 19:56
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    @JJJ The graph is not from the website. The graph here is my own and shows only Biden's poll numbers from 7/1 until now. – D_Bester Aug 13 '19 at 19:58
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    The Economist is consistently on the lower line. The Quinnipiac and CNN showed Biden at 22 immediately following the June 26/27 debates but otherwise are consistently on the higher line. Almost everyone else follows the higher line. – D_Bester Aug 13 '19 at 20:32
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What's going on is that there are two weekly polls of the Democratic race, one by Politico/Morning Consult and one by Economist/YouGov. The Economist/YouGov poll shows consistently lower support for Biden than other polls of the race.

YouGov's last six polls found Biden's support at 25, 26, 25, 25, 22, and 23. By contrast, the last six Morning Consult polls show Biden's support at 33, 33, 32, 33, 33, and 32, more in line with other polls. That's too stark and consistent a trend to be mere chance; it's clearly something to do with the methodology.

Perusing the methodology, two differences between the polls jump out at me:

  • In the most recent poll, 14% of respondents told YouGov they "don't know" who they'd vote for. But in the Morning Consult poll, voters who say they "don't know" who they're voting for are then asked to pick a candidate they are leaning towards, and those people are added to the total for that candidate.
  • The Morning Consult polls "registered voters" who plan to vote in the Democratic primary, YouGov polls "U.S. citizens" who plan to do the same.

In general, lumping in "leans" favors more established, high name-recognition candidates, and registered voters tend to be whiter, older, and more educated than the general population. So the differences are basically what we'd expect for a candidate like Biden.

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