I noticed that some polls on the RealClearPolitics site are marked with an asterisk.
What does the asterisk mean?
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Although an asterisk usually indicates that a poll is conducted by - or on behalf of - a partisan source, in the case of the RealClearPolitics tracker, this denotes that the polling includes figures for third-party candidates which have been truncated to provide the head-to-head two-party figure.
For example, looking at the most recent Trump vs. Biden General Election polls, there are three polls marked with an asterisk, CNBC/Change Research (D), NY Times/Siena, and JTN/RMG Research.
If we now switch to the most recent 4-way polls, which include figures for the Libertarian and Green party candidates, we see that these polls are the only ones included, along with an IBD/TIPP poll.
Note in particular that the IBD/TIPP poll is not marked with an asterisk on the 2-way list of polls because it provides head-to-head figures between Trump and Biden - 50/46 on the two-way image above compared with 50/45/3/1 on the listing below. The other polls do not provide head-to-head figures, and these are obtained by truncating the third-party respondents, hence the asterisks.
Unfortunately, RCP don't seem to make this clear on their website, and the only authoritative source I could find confirming this was from a 2014 Twitter conversation involving Sean Trende - one of their Senior Elections Analysts:
@SeanTrende What does the asterisk mean for certain polls listed at RCP?
@thescream61 usually that a 3rd party is included.
@SeanTrende Thanks. What other than a 3rd party will earn an asterisk? I can’t find any asterisk explanations on the site.
@thescream61 i don't do the pages, so i'm not sure. i think i've seen them in the past for other things, hence the caveat.