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
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
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
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
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%).
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.
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
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.