Tuesday, September 30, 2008

What's Wrong With the Battleground Poll?
This polling organization has been showing up in realclearpolitics average for the past week or so. The results have been curious. Here's the explanation from today's lead story on fivethirtyeight.com. Mark

I have gotten an increasing number of questions about the GWU/Battleground Poll, which presently gives John McCain a 2-point national lead, even as essentially every other current national poll shows Barack Obama with a lead of at least 5 points.

Just because a poll is an outlier doesn't necessarily mean that it's doing something wrong. Pollsters may have legitimate reasons for having a different perspective on the election, and they may also occasionally produce odd results due to chance alone.

In this case, however, the poll seems to be making a relatively fundamental mistake: it is not weighting by age.

Take a look for yourself at the "weighted tables" that Battleground released a couple of days ago (PDF). These crosstabs provide a ton of detail -- kudos to Battleground for doing so -- but unfortunately there is one red flag. This is the age makeup of their weighted sample:
18-34    17%
35-44 12%
45-64 40%
65+ 31%
Intuitively, this probably looks fairly wrong to you -- almost twice as many age 65+ voters as age18-34 voters? And in fact, it almost certainly is wrong. By comparison, here is the approximate age composition of the electorate in 2004, as according to the US Census Bureau**:
18-34    26%
35-44 17%
45-64 38%
65+ 19%
Battleground's numbers are not even close. About 19 percent of voters were aged 65 and older in 2004, as compared to the 31 percent in the Battleground sample. On the other hand, 43 percent of voters were aged 18-44, as opposed to Battleground's 29 percent. These differences are much, much too large to be attributable to chance alone. (And all of this is assuming that turnout in 2008 will match that in 2004, even though youth turnout increased markedly in the primaries and is at least somewhat probable to do so in the general election.0

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