Mark Blumenthal | September 18, 2008
My NationalJournal.com column for the week on the continuing debate over party identification, how pollsters measure it and what they should do when they see variable results is now online
will be posted later this morning.
The timing is a bit ironic. I wrote the column yesterday afternoon and then noticed this morning that the venerable CBS/New York Times poll took the highly unusual step (for them) of weighting by party ID in addition to their usual weighting procedure (emphasis added):
The combined results have been weighted to adjust for variation in the sample relating to geographic region, sex, race, marital status, age and education. In addition, the land line respondents were weighted to take account of household size and number of telephone lines into the residence, while the cellphone respondents were weighted according to whether they were reachable only by cellphone or also by land line.
Because of fluctuations in party identification, this poll was also weighted by averaging in party preferences from three recent past Times/CBS News polls.
That last line surprised me because previously, CBS and the New York Times had a policy of not weighting by party. Many casual readers of the CBS summary reports (like this one for their most recent survey) tend to assume they do, because they provide weighted and unweighted interview counts for each party subgroup. In the past, the minor partisan differences between their weighted and unweighted samples have come from their standard procedure of adjusting demographics (gender, age, race, etc.) to match Census estimates (the part described in the first paragraph quoted above).
I emailed Kathy Frankovic, the CBS News director of surveys, to ask about the decision to weight by party. Her response was that the party ID adjustment described above " is something that we have done once before in the past, when it seemed appropriate." Although I asked, she did not provide any explanation for why they deemed it appropriate this time.
The way that the CBS/New York Times pollsters chose to weight this survey is not quite the "dynamic weighting" system long advocated by Alan Abramowitz, Ruy Teixeira and others and now used by Rasmussen Reports for their national and statewide surveys, but it's close. They chose to weight to their own recent estimates of party ID rather than to results of exit polls from years past or to surveys done by other pollsters. That approach is the most defensible method, and avoids some of the potential pitfalls that I outline in my column today.
Today's CBS/New York Times release also marks the first appearance this cycle of the unique CBS/New York Times likely voter model. Rather than trying to select or screen for likely voters, the CBS/New York Times method weights voters based on their probability of turning out. I explained the procedure at length in a blog post four years ago.
PS: Pollster.com contributor Kristen Soltis made the case for weighting by party, with a focus on an earlier CBS/NYT poll.