How To Slant A Poll 101

Today, we’re going to look at two of the techniques that are often used by the mainstream media to slant polls against Republicans, both of which are on display in the latest CBS poll.

Let’s start with the sexy headline and the first paragraph of the CBS piece on the poll:

Poll: Bush Ratings At All-Time Low

The latest CBS News poll finds President Bush’s approval rating has fallen to an all-time low of 34 percent, while pessimism about the Iraq war has risen to a new high.

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Wow, sounds terrible doesn’t it? But, let’s take a look at a couple of key factors.

The first thing you have to understand is that there are 3 different groups of voters the media may poll: adults, registered voters, and likely voters. Out of these 3 categories, “likely voters” is the group that almost always turns out to be closest to the actual election results while “adults” is the group that slants most heavily towards Democrats. Although it’s difficult to pinpoint exactly how much polling “adults” instead of “likely voters” slants the poll results to the left (when compared to election results), it’s probably somewhere between 5-10 points. So, let’s split the difference and say 7.5 points.

So, it seems likely that Bush’s approval would probably be somewhere around 41.5% if this had been a poll of likely voters. Still, pretty bad.

But, there’s another factor we haven’t adjusted for: the percentage of Republicans, Democrats, and Independents who participated in the poll. In the 2004 election, the breakdown by party was as follows:

Democrats: 37%
Republicans: 37%
Independents: 26%

While those numbers can change and do change over time and there’s no set rule that says for a poll to be fair those percentages should match up exactly with the breakdown from the last election, the numbers should be pretty close.

So, let’s look at the weighted party breakdown from the CBS poll: 1018

Democrats: 37.4% (381)
Republicans: 28.4% (289)
Independents: 34.2% (348)

So, they undersampled the number of Republicans by more than 8.5% and over sampled Independents by more than 8%. Let’s adjust for that (in a very general way). Add in 8 more Republicans and we’ll say Bush’s favorability goes up 8 points. Take out 8 Independents and we’ll figure Bush loses 4 points of support (Independents were roughly split between Bush and Kerry in 2004) and now Bush’s approval rating, after having 4 points added onto it, is at 45.5. Of course, it’s not quite as simple as I’ve made it look here, nor is this as accurate as simply polling likely voters with a correct breakdown of party affiliation, but it’s close enough for our purposes.

Then, we consider the polls margin of error, 3 points, and Bush’s real approval rating among voters who’ll actually be going to the polls in November is probably somewhere roughly between 42.5 – 48.5. That’s not great, but it doesn’t have exactly the same sort of zing that 34% has either, does it?

Of course, you could make the argument that the CBS poll is just a poll of adults and it’s not meant to give people an accurate picture of how people will vote. But, if it’s just a garbage poll that doesn’t have any bearing on election results, why bother doing it in the first place? For political purposes, any poll that doesn’t use likely voters and doesn’t have a breakdown of party affiliation that’s at least roughly similar to the numbers from the last election isn’t very important or useful.

*** Update #1 ***: By the way, just to be clear about something: I do believe Bush’s poll numbers are down from last month and that this port deal is really hurting him. That’s why I’ve said that I think the port deal is fine on its merits, but that Bush should kill it for political reasons. But, this post was about showing how pollsters can distort the political landscape to the left’s advantage, not about whether Bush’s numbers are trending upwards or downwards.

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