William M. Briggs

Statistician to the Stars!

73.2% of Likely Voters Believe Poll Results

Pollster: Who will you vote for? Civilian: Oh, I don’t know; that guy, the tall one. Pollster: I’ll put you down for Obama.

The elections in Iowa are over and the results known. It’s the season of the polls! More time is spent analyzing, worrying over, and speculating about polls in the media than is spent on any other subject, such as what the candidates think about the Iraqi war. In one sense, this is understandable, because polls offer hard, quantitative data which can be “crunched” and “drilled down” into, and so on. A candidate’s opinion on the war (and on every other subject) is harder to think about, mainly because the candidates themselves tend to be as vague as they can get away with to avoid in-depth analysis.

Since so much time and effort is spent on polls, we would hope that they offer some value. So how good are these polls? Let’s look.

There are dozens of these polls done by different organizations. The leading polls, in the sense that they are quoted the most and have the biggest organizations behind them, are the Zogby and the Des Moines Register (in Iowa only, of course), so we will examine just these two, though the results are not too different for the other polls.

Here is a table of the polls by the actual results for the Iowa 2008 caucuses. I used the latest polls, taken in the day or days right before the election, not the entrance polls. These are the numbers, then, that you would use to make a guess which candidate will win, place, and show. Only the top three candidates from each party are shown. All poll data was gathered from Pollster.com. The error is the Zogby poll minus the Actual result.

Candidate Zogby Register Actual Error
Obama 31 32 37.6 -6.6
Edwards 27 24 29.7 -2.7
Clinton 24 25 29.5 -5.5
Others 12 10 3.2 +8.8
Undecided 6 9 0 +6
—————–      
Huckabee 31 32 34.4 -3.4
Romney 25 26 25.3 -0.3
Thompson 11 9 13.4 -2.4
Others 26 27 26.9 -0.9
Undecided 7 6 0 +7

The most striking thing is, regardless of party, the polls for the top three candidates under-predict the actual results. The “Others” candidates are sums of the results over all the other candidates. There are only “Undecideds” at the time of the polls and none at the time of the election when, of course, people have to actually select an actual candidate. The error is a combination of the uncertainty of what the “Undecideds” will eventually do plus error inherent in the poll itself (through biased sampling and so on).

The much larger error for “Others” candidates for Democrats is in part due to the different way the Democrat caucus is run. If, in an initial vote at a particular polling location, a candidate does not reach a minimum threshold (about 15%), then the votes for that candidate are taken away and reallocated to other candidates. So a person might have told the pollster that he was for Biden, and gone in and voted for Biden, only to have that vote taken away and given to, say, Clinton (of course, it may be he who then chooses Clinton as his second).

One thing we can tell from the Democrat caucus is that not all of the votes for the “Others” (and “Undecideds”) were re-distributed to the other candidates evenly. At the time of the vote, 12 – 3.2 = 8.8% of the “Others” were redistributed. So, too, were the 6% of the “Undecideds”. That makes 8.8 + 6 = 14.8% of the votes that were redistributed (this figure also includes the native poll error). Obama got 6.6, Edwards 2.7, and Clinton 5.5 (these are the errors). Or, stating it another way, Obama got 45% of the eligible redistributed votes, Edwards 18%, and Clinton 37%, numbers which give hints about how future elections might go once the field of candidates narrows: many more people eventually opted for Obama than the other candidates.

49% of the “Undecideds” opted for Huckabee, 4% for Romney, 34% for Thompson, and 13% went to “Others”. This again might show that there is much stronger support for Huckabee and Thompson than is generally believed. Right now, the latest Zogby New Hampshire polls have Huckabee at 10%, with Undecideds at 8%; these numbers were taken before the Iowa results. McCain and Romney are a little over 30% each. So my guess is that by the time the votes are in from New Hampshire, it’ll be fairly even between McCain, Romney, and Huckabee, the results being in that order.
Of course, some of the error is due to the polls themselves, and, using error results from polls in previous presidential elections, I predict that we will see this error actually increase as the number of candidates shrinks. New Hampshire is less than a week away, so we’ll soon see, as two of the Democrat candidates have already dropped from the race.

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