Alan Abramowitz writes:
In five days, Clinton’s lead increased from 5 points to 12 points. And Democratic party ID margin increased from 3 points to 10 points.
No, I don’t think millions of voters switched to the Democratic party. I think Democrats are were just more likely to respond in that second poll. And, remember, survey response rates are around 10%, whereas presidential election turnout is around 60%, so it makes sense that we’d see big swings in differential nonresponse to polls which will not be expected to map to comparable swings in differential voting turnout.
We’ve been writing about this a lot recently. Remember this post, and this earlier graph from Abramowitz:
and this news article with David Rothschild, and this research article with Rothschild, Doug Rivers, and Sharad Goel, and this research article from 2001 with Cavan Reilly and Jonathan Katz? The cool kids know about this stuff.
I’m telling you this for free cos, hey, it’s part of my job as a university professor. (The job is divided into teaching, research, and service; this is service.) But I know that there are polling and news organizations that make money off this sort of thing. So, my advice to you: start poststratifying on party ID. It’ll give you a leg up on the competition.
That is, assuming your goal is to assess opinion and not just to manufacture news. If what you’re looking for is headlines, then by all means go with the raw poll numbers. They jump around like nobody’s business.
P.S. Two questions came up in discussion:
1. If this is such a good idea, why aren’t pollsters doing it already? Many answers here, including (a) some pollsters are doing it already, (b) other pollsters get benefit from headlines, and you get more headlines with noisy data, (c) survey sampling is a conservative field and many practitioners resist new ideas (just search this blog for “buggy whip” for more on that topic), and, most interestingly, (d) response rates keep going down, so differential nonresponse might be a bigger problem now than it used to be.
2. Suppose I want to poststratify on party ID? What numbers should I use? If you’re poststratifying on party ID, you don’t simply want to adjust to party registration data: party ID is a survey response, and party registration is something different. The simplest approach would be to take some smoothed estimate of the party ID distribution from many surveys: this won’t be perfect but it should be better than taking any particular poll, and much better than not poststratifying at all. To get more sophisticated, you could model the party ID distribution as a slowly varying time series as in our 2001 paper but I doubt that’s really necessary here.