August 20, 2008
By Brian Schaffner
On Monday, CBS News and the New York Times released a survey of delegates attending the Democratic National Convention in Denver next week. The major finding from this survey came from a question asking delegates who they would like to see as the vice presidential candidate. 28% of the delegates interviewed preferred Hillary Clinton, compared to just 6% who selected Joe Biden (the second most popular choice). The support for Hillary Clinton attracted a lot of attention from news outlets, as well as Pollster.com readers. Of particular interest is whether the 28% figure for Clinton is particularly high or low. To answer this question, we could use a little historical context.
I was able to dig up two earlier Democratic delegate surveys which asked similar questions about VP preferences--one from 1988 and the other from 1992. The results from these surveys are presented in the table below. I've shown the top five finishers in each survey, as well as the percentage naming another candidate ("Other") and the percentage declining to name anyone ("no preference").
The first important point that stands out from this table is that support for Clinton is almost twice as high as it was for any other single candidate in 1988 or 1992. Bill Bradley had the support of 15% of Democratic delegates in 1992, while Jesse Jackson was the preferred candidate for 14% in 1988.
The second notable pattern from 1988 and 1992 is that the eventual VP pick was not one that delegates named in large numbers before the convention. In 1988, only 2% of convention delegates mentioned Lloyd Bentsen as a running mate for Dukakis while just 5% recommended Al Gore in 1992. Thus, Clinton's standing in first place does not necessarily bode well for her chances of ending up on the ticket.
Third, notice that VP ambivalence is not a new phenomenon in 2008, nor is the phenomenon of having a wide swath of politicians named. In 1988, one-third of convention delegates declined to state a preference for Dukakis's running mate, while one-quarter of delegates did not name anyone in 1992. In both years (along with 2008) support was scattered across dozens of names, with only one or two candidates even breaking double-digits in any given year.
Finally, I was able to get the raw data for the 1992 convention delegate survey to look at one additional question: to what extent do a losing candidate's delegates promote their candidate for VP? In 1992, Jerry Brown and Paul Tsongas lost out on the nomination; but each candidate sent plenty of committed delegates to the Democratic convention. The figure below shows who those delegates preferred for Clinton's VP choice.
The figure reveals mixed patterns. Interestingly, Jerry Brown's supporters picked plenty of different possibilities for a Clinton running mate, but almost none of them supported Brown himself. Tsongas's supporters, on the other hand, were more likely to name Tsongas as a potential running mate, but they were also about as likely to name Bill Bradley. For their part, (Bill) Clinton delegates were the most ambivalent, with one-third of them failing to state a preference.
Overall, the historical comparison reveals that Clinton's support is high compared to delegate preferences in 1988 and 1992. However, pre-convention support among delegates didn't do much for Jesse Jackson in 1988 or Bill Bradley in 1992.
By Brian Schaffner on August 20, 2008 2:01 PM
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August 13, 2008
By Brian Schaffner
As Pollster.com readers have no doubt noticed, there has been much discussion in the posts and the comments here about the merits of polling registered voters (RV) versus likely voters (LV). Mark and Charles have been debating this point in their most recent exchanges about whether it is better to include LV or RV results in the Pollster.com poll averages. Charles's last post on this topic raised the following questions:
"There is a valid empirical question still open. Do LV samples more accurately predict election outcomes than do RV samples?"
Ideally, I'd have time to go back over 30 or more years of polling to weigh in on this question. Instead, I thought I'd go back to 2004 and get a sense of how well RV versus LV samples predicted the final outcome. To do this, I used the results from the final national surveys conducted by eight major survey organizations. For each of these eight polls (nearly all of which were conducted during the last three days of October), I tracked down the Bush margin among both RVs and among LVs. The figure below demonstrates the difference in the Bush margin for the LV subset relative to the RV sample from the same survey.
For most polls, LV screens increased Bush's margin, including three surveys (Gallup, Pew, and Newsweek) where Bush did 4 points better among LVs than he did among RVs. But using a LV screen did not always help Bush. In three polls, (CBS/New York Times, Los Angeles Times, and Fox News) his margin remained the same and in the Time poll (which was conducted about a week earlier than the other surveys) Bush actually did 2% worse among LVs.
Of course, this doesn't really tell us which method was more accurate in predicting the general election outcome, just which candidate benefited more from the LV screens. To answer which was more accurate, we can plot each poll's Bush margin among both RVs and LVs to see which came closest to the 2.4% margin that Bush won in the popular vote. This information is presented in the figure below, which includes a dot for each survey along with red lines indicating the actual Bush margin.
Presumably, the best place to be in this plot is where the red lines meet. That would mean that both your RV and LV margins came closest to predicting the eventual outcomes. But, if you are going to be closer to one line over the other, you'd rather be close to the vertical line than the horizontal line. This means that the polling organization's LV screen helped them improve their final prediction over just looking at RVs. If the opposite is true (an organization is closer to the horizontal line than they are to the vertical line), their LV screen actually reduced their predictive accuracy.
The CBS/New York Times poll predicted a 3 point Bush margin for both its RV and LV samples, meaning it was just 6/10ths of a point off regardless of whether they employed their LV screen. Four organizations (Pew, Gallup, and ABC/Washington Post, and Time) increased the accuracy of their predictions by employing the LV screens, coming closer to the vertical line than they do to the horizontal line. Gallup's LV screen appeared to be most successful, since it brought them closest to the actual result (predicting a 2 point victory for Bush despite the fact that their RV sample showed a 2 point advantage for Kerry).
On average, the RV samples for these eight polls predicted a .875 Bush advantage while the LV samples predicted a 2.25 advantage for Bush, remarkably close to the actual result. Of course, this is just one election, but it does appear as though likely voters did a better job of predicting the result in 2004 than registered voters. On the other hand, this analysis reinforces some other concerns about LV screens, the most important of which is the fact that some LV screens created as much as a 4 point difference in an organization's predictions while in three cases LV screens produced no difference at all. It is also important to note that these are LV screens employed at the end of a campaign, not in the middle of the summer, when it is presumably more difficult to distinguish LVs. Ultimately, the debate over LV screens is an important one and the 2008 campaign may very well provide the biggest challenge yet to pollsters trying to model likely voters.
By Brian Schaffner on August 13, 2008 4:42 PM
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July 29, 2008
By Brian Schaffner
"We have a race that by every measure of every poll is a statistical dead heat. McCain's not supposed to be in this thing, and Obama's supposed to be blowing everybody away and it just isn't happening, at least to this point."
Lou Dobbs (July 17th, Lou Dobbs Tonight)
If you have paid any attention to the news in the past month, you have had a hard time avoiding some journalist or pundit noting that the presidential race is currently a "statistical dead heat" or "essentially tied." The news media, of course, love to cover the horserace aspects of the campaign, particularly in a way that emphasizes how close the election is. But when you step back and gain a little perspective on the big picture, you realize that this race isn't quite the dead heat that it is made to be.
The news media are often a bit myopic in their view of the contest, extrapolating too much from the most recent poll (or even the most recent "surprising" poll). Last week, Fox News released a national survey that showed Obama holding a 41-40% lead, well within the margin of error for the survey. Commentators were quick to emphasize this result and note that the candidates were essentially running neck-and-neck or that the race may even be tied. No doubt there will be a lot of commotion over the latest Gallup/USA Today survey showing McCain ahead (though also within the margin of error) among likely voters. Nevertheless, we gain much better perspective on the state of the race when we look at all available data.
Alan Abramowitz notes that Obama has consistently led in national polls over the past two months. In fact, according to national poll results listed on Pollster.com, Obama had been tied or ahead in 50 consecutive national polls through Sunday. Sure, many polls may show Obama holding a lead within the statistical margin of error, but if Obama and McCain were actually tied, we'd expect as many polls showing McCain ahead as show Obama ahead. Based on some basic calculations, the probability that 50 consecutive national surveys would show Obama tied or ahead if the candidates were actually tied is .0000000000000009. In short, this race is not a "statistical tie," despite what a few scattered surveys (drawing disproportionate attention from the pundits) indicate.
By Brian Schaffner on July 29, 2008 2:45 PM
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