Why Polls Don’t Match Up in Presidential Race

October 24, 2008 at 12:13 am Leave a comment

obama-mccain-23.jpegBarack Obama is galloping away with the presidential race. Or maybe he has a modest lead. Or maybe he and John McCain are neck and neck.

Confusing? Sure, thanks to the dueling results of recent major polls.

In
the past week, most surveys have shown Democrat Obama with a
significant national lead over Republican McCain. Focusing on “likely
voters” — as many polling organizations prefer this close to Election Day
— an ABC News-Washington Post survey showed Obama leading by 11
percentage points. A Wall Street Journal-NBC News poll had the same
margin, while the nonpartisan Pew Research Center gave Obama a 14-point edge.

But
others had the race much closer. CNN-Opinion Research detected an Obama
lead of 5 points. The George Washington University Battleground Poll
had Obama up by 4 points. And an Associated Press-GfK poll showed Obama
at 44 percent and McCain at 43 percent — in effect, a tie.

How can this be? Some questions and answers about why the polls differ.

Q: Don’t pollsters simply ask questions, tally the answers and report them?

A:
No. After finishing their interviews — usually with about 1,000 people,
sometimes more — they adjust the answers to make sure they reflect Census Bureau data
on the population like gender, age, education and race. For example, if
the proportion of women interviewed is smaller than their actual share
of the country’s population, their answers are given more “weight” to
balance that out. But some pollsters make these adjustments differently
than others. And while most polling organizations including the AP do
not modify the responses to reflect some recent tally of how many
Democrats, Republicans and independents there are, some do.

Q: Are those the only changes made?

A:
No. As Election Day nears, polling organizations like to narrow their
samples to people who say they are registered voters. They often narrow
them further to those they consider likely voters. That’s because in a
country where barely more than half of eligible voters usually show up
for presidential elections, pollsters want their polls to reflect the views of those likeliest to vote.

Q: Is that hard to do?

A:
Quite hard, since no one will truly know who will vote on Election Day
until that day is over. In fact, virtually every polling organization
has its own way of determining who likely voters are.

Like
many polling organizations, the AP asks several questions about how
often people have voted in the past and how likely they are to vote
this year, and those who score highest are considered likely voters.

Q: Why is this such a problem?

A:
Because nobody is 100 percent sure how to do this properly. And the
challenge is being compounded this year because many think Obama’s
candidacy could spark higher turnout than usual from certain voters,
including young voters and minorities. The question pollsters face is
whether, and how, to adjust their tests for likely voters to reflect
this.

In identifying likely voters, the AP does not build in an assumption of higher turnout by blacks or young voters. Pew Director Andrew Kohut says that reflecting exceptionally heavy African-American turnout in the Democratic primaries, Pew’s model of likely voters now shows blacks as 12 percent of voters, compared to 9 percent in 2004.

Underscoring the uncertainty, the Gallup Poll
is using two versions of likely voters this year — a traditional one
that asks about peoples’ past voting behavior and their current voting
intentions; and an expanded one that only looks at how intent they are
on voting this year, which would tend to include more new voters.

Q: What else might cause differences?

A: The groups pollsters randomly choose to interview are bound to differ from each other, and sometimes do significantly.

Every poll has a margin of sampling error,
usually around 3 percentage points for 1,000 people. That means the
results of a poll of 1,000 people should fall within 3 points of the
results you would expect had the pollster instead interviewed the
entire population of the U.S.
But — and this is important — the results are expected to be that
accurate only 95 percent of the time. That means that one time in 20,
pollsters expect to interview a group whose views are not that close to
the overall population’s views.

Q: Are the differences among polls this year that unusual?

A: Not wildly, but that doesn’t make them less noticeable.
There’s a big difference between a race that’s tied in the AP poll, and
Pew’s 14-point Obama lead. But because of each poll’s margin of error, those differences may be a bit less — or more — than meet the eye.

That’s because each poll’s margin of sampling error should really be
applied to the support for each candidate, not the gap between them.

Take the AP poll, which has a margin of error of plus or minus
3.5 percentage points. Obama’s 44 percent support is likely between 48
percent and 40 percent. McCain’s 43 percent is probably between 47
percent and 39 percent.

When support for candidates is measured in ranges like that, some polls’ findings could overlap — or grow worse.

Q: Are people always willing to tell pollsters who they’re supporting for president?

A: No, and that’s another possible source of discrepancies. Some
polling organizations gently prod people who initially say they’re
undecided for a presidential preference, others do it more vigorously.
The AP’s poll, for example, found 9 percent of likely voters were
undecided, while the ABC-Post survey had 2 percent.

Source: AP

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