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Why the post-debate polls for CNN and YouGov are wrong

Donald Trump will not win a poll with a +10% Democratic voter sample, and pollsters know it

After a strong debate that will likely save Trump's poll numbers, Conway sent a warning to Paul Ryan | photo

The YouGov poll, to its credit, provided internals for their poll. CNN has not, although apparently there is more to come.

According to the YouGov poll, Clinton edged out Trump in the debate last night by a 47 to 42 margin. The CNN poll, absurdly, found that Clinton won the debate by a twenty-three point margin, 57 to 34. CNN notes in its article that their results match voter preference from before the debate, where 58% of the debate watchers they are surveying support Clinton.

Sadly, some analysts that people take seriously are lending credibility to these polls, most notably Nate Silver. These analysts shouldn't attach any significance to these polls because they make two fundamental mistakes.

The YouGov poll surveyed registered voters and did not apply a rigorous likely voter screen. They included all voters who did not say that they "definitely" will not vote. Polls of registered voters generally favor Democrats, as even Nate Silver acknowledges. Apparently CNN did not even limit its poll to voters at all, but all debate watchers. Did they poll children? Possibly.

Daily Oct. Surprise tracker 

  October Surprise After 1 day After 2 days After 3 days After 4 days After 5 days After 6 days After 7 days
National Shift --- 1.45 to Clinton 3.4 to Clinton 3.08 to Clinton 3.09 to Clinton 2.51 to Clinton    

See the full results here

The second big mistake that YouGov made was not adjusting its voter sample to reflect the population at large. The poll assumes that Democrats would outnumber Republicans in this election by 10%. Of course, if that's the turnout, Democrats will win in a landslide.

Because CNN did not bother to limit its poll to voters, the network would have no need for a turnout model.

In summary, the CNN poll is not a political poll but something more like a television analysis. It's just not useful in a political context and shouldn't be cited.

YouGov did an actual political poll, but did not apply a real likely voter screen and heavily oversampled Democrats. Garbage in, garbage out. IGNORE THESE POLLS.

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