10/04/2007 11:25 pm ET | Updated May 25, 2011

ABC/Post Poll: Surge or Outlier?

About that new ABC
News/Washington Post poll
yesterday, the one showing Hillary Clinton winning the support of 53% of "leaned
Democrats" nationally to 20% for Barack Obama, 13% for John Edwards and single
digits for the other candidates. Since this survey was the first to show a majority
supporting Clinton, a lot of bloggers let loose with the adjectives, writing that
she "pulls
" with a lead that is "crushing" or "explodes," telling us that we
have reached "knockout
punch time
," that "she's killin' it,"
or declaring simply, "oh
baby, when she moves, she moves
." My friend Chris Cillizza parsed Clinton's "surge"
in his blog and concluded:

It doesn't take a polling expert to
understand that Sen. Hillary Rodham Clinton's showing in the latest Washington
Post/ABC News poll is a major moment in the campaign.

There is obviously no denying that the poll has created the perception of a major change in the
race, but has a Clinton really experienced a "surge" of support over the last
month, as indicated by the 12-point increase (from 41% to 53%) in her support
on the Post/ABC poll? I'm not so sure.

I checked our national chart for the Democrats late
yesterday and saw that this new ABC/Post poll appears to be very different from
the others conducted over the last month. I quickly realized, however that this
particular comparison highlights a frustrating limitation of our chart, which
is that about half of the national polls plotted include Al Gore as an option
and about half do not. Since Clinton typically picks up roughly five points on
polls - including the new ABC/Post survey - that do not offer Gore as a choice,
our standard chart may make the ABC/Post survey look like more of an outlier
than it is.

So we decided to devote some time today to producing a chart
that displays only the results for polls without Gore (using recalculated votes
based on second choice when available). The result below plots 66 national polls
released so far this year (about a third fewer than our standard version - as not
all provide a "vote without Gore"):


The chart makes a few things clear. First, the 53% result for new Post/ABC poll is more of an "outlier"
from the regression trend line than any poll conducted this year (it's the purple dot at the extreme top right of plot area). At 53%, the
polls estimate of Clinton's
support falls a full ten percentage points higher than our current estimate of
the trend (42.5%) even without Gore in the race.

Second, the addition of the new poll has not budged our
trend line. That is mostly by design. Professor Franklin set the sensitivity of
the regression model
to prevent the inevitable outlier from causing wild gyrations
in the trend line. That is one of the reasons we prefer to use regression
estimates over simple rolling averages.

One of the reasons we put so much effort into these plots is
that they paint a picture of the random variation that is inherent in polling. You
can actually see the random variation. While it typically appears as a random and
predictable "cloud" of points, bigger outliers still occur from time to time. The
more experience you have looking at poll numbers, the more you learn to
understand that even with well designed polls, outliers happen. Our advice to poll
consumers - as reflected in the design of the charts -- is to try to avoid over-reacting
to any one poll.

Of course, we do not yet know whether this poll is really a
statistical outlier. Other polls have obviously been showing a more gradual
increase in her support recently, and it is still theoretically possible that Clinton's
support suddenly lurched up ten points last week. Senator Clinton had appeared
on all of the Sunday morning talk shows on September 23, just four days before
this poll went into the field. Earlier that week sho also rolled out her plan
to reform health care, an issue that ABC/Post and other polls confirm as one her
great perceived strengths. But a sudden upward surge of this magnitude does not
seem very likely, if only because no other news event so far this year has caused
anywhere near as much change in the Democratic race.

We should know soon enough. Unfortunately, the AP-IPSOS
released yesterday does not help resolve this issue, as Clinton's Sunday talk show
tour de force occurred in the middle of their field period and (I'm told) after
they had completed most of the interviews on the main sample.

So we will wait and see. But I'll wager that a month from
now the real trend will not look nearly as dramatic as the one suggested by yesterday's

PS: It will probably take a few days, but we will have a
full, regularly updating version of the national "vote without Gore" up soon.

UPDATE (10-16): More recent polls do move the Clinton trend line up and make the Post/ABC poll look much less like an outlier. More here.