The Recurring -- and Misleading -- Focus on Party Identification
The discussion of the party identification composition of poll samples comes up in every presidential election with which I've been involved. Interested observers often opine that when a given poll shows that Candidate X is ahead, it cannot be correct because there is a higher percentage of voters who identify with Candidate Xs party in the sample than there should be, based on comparison to some previous standard.
There are several reasons why this is a faulty approach to evaluating a poll's results.
Party identification is basically an attitudinal variable, not a stable population parameter. It is designed to vary. This is distinct from demographic variables such as age, gender, race/ethnicity, and education, which are, generally speaking, stable indicators measured by the U.S. Census Bureau. The only issues relating to demographic variables are measurement concerns -- e.g., how the census, which creates the targets, measures ethnicity versus how individual pollsters measure it. But, generally speaking, these are fairly stable targets.
Party identification is not measured by the U.S. Census Bureau, nor are there any other official state or national standards for what party identification "should be" in terms of the percent per party as it relates to the general population.
Many people use the exit polls as a standard. But exit polls use a distinct question wording, a different methodology (in person interviews at the polling place as opposed to telephone interviews), a different environment (people are asked their party identification just after having voted, which could affect how they answer), and different sampling techniques to develop who it is that is asked the question. So party identification figures as measured by a specific poll aren't easily compared to party identification as measured by an exit poll because of these and other potential issues.
Party identification changes as political tides change. General shifts in the political environment can affect party identification just as they can affect presidential job approval and results of the Who are you going to vote for? question.
Here is how Gallup asks party identification: In politics, as of today, do you consider yourself a Republican, a Democrat, or an independent?
Note that this question does not ask, What was your party identification in November 2008? Nor does it ask, Are you registered with one party or the other in your state? Our question uses the words "as of today" and "consider." It is designed to measure fluidity in political self-identification.
We know that party identification moves over time -- sometimes in very short periods of time, just like other political variables. Generally, if there is a political tide toward either of the two major parties, all questions we ask that are of a political nature will move in that direction. This includes the ballot, job approval, party identification, among others.
So, it would not be surprising to find that if Barack Obama is enjoying a surge in popularity in any given state, that surge will show up on the ballot question, on his job approval measure, and on the measure of party identification. So, data showing that Obama is ahead on the ballot in a specific state poll and that Democrats have a higher-than-expected representation on the party identification question, are basically just reflecting two measures of the same underlying phenomenon.
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