Bottom line: The devil is indeed in the details.
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Let’s get really numbers geeky today ….
To satisfy, my curiosity, I dug a bit deeper into the 12 point difference between the recent Washington Post – ABC Poll (Clinton +12) and the historically accurate IBD poll (race tied).
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Avid Trump supporters claim that WaPo over-sampled Dems … boosting Hillary’s numbers.
Break that into 2 parts: (1) Did WaPo over-sample Dems? (2) More generally, does over-sampling matter?
First, let’s draw a distinction between “over-sampling” and “over-weighting”.
Over-sampling means surveying more people in a particular group than their proportionate share of the population.
Over-weighting is disproportionately counting folks in a particular group when compiling an overall total.
Specifically, IBD says that it polls Democrats, Republicans and Independents in roughly the same numbers.
In other words, they over-sample Republicans and Independents since more than 1/3 of likely voters are Democrats (or, so they say).
But, IBD corrects for the over-sampling by re-weighting based on population proportions derived by separate studies called “enumeration surveys”.
Specifically, the IBD poll weights Democrats 36%, Republicans 30% and Independents 34%, assuming that mix is representative of likely voter turnout.
Note: overall, headline total numbers are highly sensitive to these turnout assumptions. Since roughly 85% of folks typically vote for their party’s nominee, each point shift in the turnout assumptions changes the weighted average by almost a point. And, these turnout ratios are derived outside of surveys based on the enumeration studies and political analysis.
As near as I can tell, the WaPo turnout weightings are about the same as IBD’s … maybe a couple of points more skewing to the Dems … maybe accounting for 2 or 3 of the 12 points.
No big deal.
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Digging deeper is where things start to get really interesting.
The Homa Files 