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Obama-Clinton "Virtually Tied" or Not?

Topics: 2008 , Divergent Polls , The 2008 Race

The poll story of the day is the latest from USA Today and Gallup that we linked to a few hours ago which, to quote the USA Today story, shows Hillary Clinton and Barack Obama "essentially tied for the Democratic presidential nomination...the first time that the New York senator hasn't clearly led the field." Why does the Gallup poll show this seemingly dramatic change while other recent surveys show Clinton maintaining a wider lead?

Let's start with this important hedge from Gallup:

In a technical sense, neither Clinton's drop of 6 percentage points nor Obama's gain of 4 points are statistically significant, given the relatively small sample size of less than 500 Democrats and Democratic-leaning independents interviewed in each poll.

Because the changes are within the poll's margin of sampling error, it is unclear if Clinton has ceded her front-running status for co-front-runner status -- future polling will help confirm that.

Given the smaller sample sizes, we should expect to see the sort of poll-to-poll movement evident on the recent Gallup survyes, as pointed out in a blog post today by Gallup's Frank Newport. As such, I find it a little surprising that Newport also chooses to "characterize the Democratic race as one of stability marked by the occasional burst of change." The "occasional burst" seems more a function of sampling variability than the race.

Second, another question we might want to ask is whether this narrower margin was the result of a change in the demographic composition of the Democrats and Democratic leaning independents surveyed. For example, most polls have shown Clinton running better among women, older voters, the less well-educated and Democratic identifiers (as opposed to "leaners"). Demographic composition can also show random variation. So did this poll include smaller percentages of Clinton best groups than previous Gallup polls? Unfortunately, the Gallup and USA Today reports, like those of most other media outlets, provide little or no information about the demographic profile of those who answer primary vote questions.

Susan Page's USA Today story does tell us that most recent sample consists of "of 310 Democrats and 160 independents who 'lean' Democratic" and then goes on to quote Clinton pollster Mark Penn disparaging the poll for its inclusion of independents:

Mark Penn, Clinton's chief strategist, calls the USA TODAY poll "an outlier" that is "completely out of sync" with other surveys. He says it is "seriously flawed" for including so many independents unlikely to vote in Democratic primaries.

However, the methodology of the USA Today/Gallup poll has not changed since May, when Clinton led and Penn apparently had no such complaints, at least none that made it into write-up. The issue is whether this survey includes more independents than previous polls by USA Today and Gallup. Unfortunately, if either organization has released the percentage of independent leaners included in previous Democratic primary results, I have not seen it. Page does go on to report (as noted by Andrew Sullivan):

Among Democrats alone, Clinton leads Obama by 5 points, 34%-29%. That's a significant narrowing from the USA TODAY Poll taken in mid-May, when she led by 17 points. Among independents, Obama leads by 9 points, 31%-22%.

Rather than obsessing over this one survey of 470 respondents, readers might want to take a longer view. First, we see a less dramatic but statistically significant narrowing of the race if we average the results from the last four Gallup surveys (conducted since late April) and compare them to the four Gallup surveys before that (conducted between February and early April). In the February-to-April polls, Clinton led Obama by 16 points (37% to 21%). On the last four Gallup polls, she leads Obama by just seven (33% to 26%).

06-04%20usa%20today%20gallup.png

A chart of the results shows that while Clinton's share of the vote has been fluctuating (again, not surprising given the sample sizes), Obama's support has been higher on all four of the most recent polls than on the four before that.

06-04%20chart_sml.png

Finally, but perhaps most importantly, consider our own chart that summarizes the trend on all of the national horserace results conducted to date. While we see little or no change in Obama's number since the upward movement following his announcement, Clinton's number has shown a very slight decrease (roughly 2-3 points) since February. The chart displayed here does not yet include today's Gallup numbers, but I will leave it to Professor Franklin to interpret this recent twitch in the National Democratic numbers.

ATopDems400.png

UPDATE: USA Today Polling Editor Jim Norman emails with more information on how this survey compares to the most recent Gallup polls in terms of demographics and the proportions of independents.

 

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