Update: Since this article was published, NBC has announced that among responses after the Sunday night debate, Clinton's lead shrank to a 7-point advantage in both a four-way matchup and in a head-to-head race -- reflecting the same margin that Clinton showed in a mid-September NBC/WSJ poll." In other words, Clinton holds 7-point lead in a poll with 16% more Democrats interviewed and (in this poll) has recovered from any presumed damage over the lewd remarks.
Original article follows:
Democrat Hillary Clinton has doubled her lead over Republican Donald Trump in a new WSJ-NBC News poll. It's good news for Clinton and bad news for Trump supporters after the release of a recording of his lewd, private conversation from years ago.
But that may not the full story.
In focus on Full Measure this past Sunday, we heard from a polling expert who cautioned that the headline of any poll rarely tells the full story. Looking at the sample, the WSJ-NBC News poll, like many others, leaned more heavily on Democrats. If Democrats turn out in significantly greater numbers than Republicans, then the poll is likely to be most accurate. If not, then it may not be a true measure of the state of play. On the other hand, there's little doubt that Trump's popularity has slipped when compared to similar, previous Democrat-heavy polls.
Another factor introducing possible bias in polls, according to our expert, is the types of questions asked and they way they are asked. For example, respondents in the WSJ-NBC News poll were queried about Trump's "vulgar" comments. But it appears they were not asked about any negative Clinton developments, such as Wikileaks emails showing that, in paid speeches, she privately told Wall Street types that she has a "private" position on issues and a different "public position," and used the phrase "open borders."
In the WSJ-NBC News poll sample of 500:
43% or 215 people identify as Democrats
36% or 180 people identify as Republicans
So 35 (16%) more Democrats were interviewed than Republicans.
[quote]Therefore, Clinton is leading 10 percentage points among likely voters in a sample that interviewed 16% more Democrats.[/quote]
13 percentage points more of the sample voted for Obama over Romney in 2012 (yet Obama won by less than 4 percentage points, indicating this poll may lean much more heavily toward Democrats).
7 percentage points more of the sample want a Democrat-controlled Congress, further indicating a Democrat-heavy poll.