We have been busy here over the last day or two, including links to 8 new polls that interviewed through Sunday night, so I am going to try use this post to wrap things up a bit. All but one of the late surveys shows Clinton leading by margins of 5 to 13 points, so to no one's surprise, most expect Hillary Clinton to defeat Barack Obama tonight. The suspense seems to be about the size of Clinton's margin. On that score, unfortunately, the polls are not conclusive.
Why not? Here's the short version: (1) The pattern of smaller undecideds correlating with larger Clinton margin has largely disappeared over the last week, (2) tracking polls have been inconsistent about late trends and (3) the ultimate margin will depend on how well these surveys have selected likely voters. The longer version follows:
1) Do undecideds look like Clinton voters?
Maybe, maybe not.
The notion of undecided voters "breaking" to one candidate or another is something of a misnomer to begin with. It makes the implicit assumption that all surveys measure the true electorate and that all voters that express preferences on surveys are truly decided, leaving the final margin in the hands of voters that tell pollsters they are "undecided."
In reality, our "likely voter" models inevitably include some adults that end up not voting and exclude some that do. As such, the "undecided" category on the final round of polling usually includes a disproportionate share of those who are disengaged from the race and end up not voting. Also, some voters tell pollsters they have a preference even though they say they may still change their minds (9% of those with a preference in Pennsylvania, according to the final Mason-Dixon survey).
A week ago, I used my National Journal column to highlight a pattern in the surveys that suggested a hidden vote for Clinton. The Obama percentage appeared relatively stable across polls while the Clinton percentage varied considerably with the size of the undecided category. As the undecided percentage decreased, Clinton's percentage grew.
On the last round of polls, however, the pattern that I highlighted has disappeared. I updated the chart used in the column with the polls fielded over the final weekend highlighted in dark blue. The wide "spread" in the dots is gone. The previous pattern had owed largely to differences in the results from two pollsters (which both use an automated methodology): SurveyUSA showed big Clinton leads and small undecided, while Public Policy Polling (PPP) showed Obama even or slightly ahead and a larger undecided percentage.
On the last round of surveys, two important things changed. SurveyUSA, still finding very few "undecided" voters, showed the Clinton margin narrowing significantly, while PPP added a follow-up question asking undecided voters how they lean. PPP continues to show Obama with a slight lead, only with a much smaller undecided percentage. So the pattern of dots in the chart is now more circular, and the relationship between the size of the undecided category and the Clinton margin has all but disappeared (something Poblano also noticed yesterday).
Of course, the remaining undecided may still conceal a disproportionate share of Clinton voters, but hard evidence of that proposition is weak. Chuck Todd noticed that undecided voters in the MSNBC/Mason-Dixon survey were higher in subgroups where Clinton does better (among gun owners and outside of the Pittsburgh and Philadelphia media markets). Looking back at theTime/SRBI survey conducted in early April, Charles Franklin saw evidence that undecideds seem "somewhat more likely to support Clinton." However, as I look at the pattern of undecideds in the most recent SurveyUSA and Quinnipiac surveys, I see no clear pattern in the undecided either by region or demographic subgroups. On the Quinnipiac survey, for example, the percentage of undecided voters is roughly same among African Americans (6%) and white voters without a college education (5%).
Gallup's Frank Newport looked at the evidence on this question last Friday (a post worth reading) and "neatly" concluded that "undecideds either will or will not break for Clinton in Pennsylvania." That's about right.
2) Are polls showing a late trend?
Once again, unfortunately, the bottom line is maybe, maybe not.
The Zogby rolling average tracking shows the Clinton margin growing from one percentage point (46% to 45%) to ten (51% to 41%). However, other surveys that have tracked twice over the last week to ten days show no consistent trend. As the table below shows, both SurveyUSA and ARG showed essentially a trend in Obama's favor over the last week, while four other pollsters showed essentially no change. On average, these "apple-to-apple" comparisons show Clinton's percentage increasing by less than a point, Obama's by roughly two. Ignoring statistical significance, four polls showed movement in Obama's direction, two showed movement in Clinton's direction and one showed no change in the Clinton-Obama margin.
Click the thumbnail below to see a larger version with more complete data:
Our trend estimates add another wrinkle. The standard trend lines look parallel, suggesting little or no change in Clinton's 6-7 point margin over the last week. However, as Charles Franklin explained earlier this morning, the more sensitive estimate -- which gives greater weight to more recent polls (including a few that had not been tracking a week ago) -- shows a slightly bigger Clinton margin (8.4 points).
So, again unfortunately, we either have evidence of a late trend, or we do not.
The exit polls tonight will help resolve whether late deciders have have favored one candidate. Yesterday, drawing on recent exit polls, ABC's Gary Langer noted:
Late deciders have been turning to Clinton recently, but only recently. She’s done better among late deciders than among other voters in 11 contests, including the eight most recent. Going farther back, though, she’s done the same among late deciders in 10 contests, and worse in 10.
3) "It's the Turnout Stupid"
That's the way FiveThirtyEight's Poblano put it yesterday, and he's right. For all our worry about late shifts and the problems of interpreting the "undecided" category, the collective accuracy of the polls (or lack thereof) in predicting Clinton's margin probably depends even more on how well they have done selecting "likely voters."
Give a pollster 1,000 voters to interview, and our measures do a reasonable job discerning their preferences. But trying to discern the actual primary voters from a random sample of 1,000 adults is not so easy and far less accurate. Different methods of selecting "likely voters" can end up selecting different kinds of people. Since the Obama-Clinton race features large differences in vote preference by race, gender, age, socio-economic status and region, relatively small shifts in the composition of the electorate can alter the vote margin noticeably.
As I reviewed yesterday, if we look at their composition in terms of race, age, gender, and years of education, the Pennsylvania polls show meaningful variation. Given the demographic patterns in the vote, a difference of four points in the African-American contribution on most polls can lead to a three point shift in the Clinton-Obama margin. Differences of five percentage points in terms of the contribution of white voters under 35 or white voters with a college education may translate into two-point shifts in the Clinton-Obama margin. The same is probably true for the share of the vote in the Philadelphia metro area (as Virginia Centrist points out).
"I want to know the future," Pollster reader Fourth wrote yesterday. "Is that too much to ask?"
No, it's not. Unfortunately the challenge of selecting likely primary voters is what makes these pre-election polls blunt instruments as predictors. They can give us a general sense of where things stand, which way they are moving (when the movement is large) and guidance about what each candidate needs to do to maximize their support. But the problem involves too many unknown variables to try to predict the outcome with precision.
The future will be here in about 12 hours. We will know soon enough.
[Embarrassing typos repaired. Many thanks to Pollster reader JB for his unsolicited fill-in for Eric].