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Mark Blumenthal

Mark Blumenthal

Posted: May 2, 2007 03:09 PM

The Field Poll and RBS Sampling

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One of the more intriguing things included in last week's post on recent polling in California is the shift by the state's venerable Field Poll to voter registration list sampling for their pre-election surveys.

The debate over the use of sampling from lists, rather than randomly generated telephone numbers continues to rage among pollsters. We had a guest post by Democratic pollster Amy Simon late last year which addressed the key issues. The gist is that most media pollsters continue to rely on "random digit dial" (RDD) samples, while campaign pollsters have largely shifted to sampling from registered voter lists. The media pollsters argue that RDD samples are superior because the randomly generated telephone numbers allow coverage of all telephone households, including unlisted numbers. The campaign pollsters argue that computerized lists have improved in recent years, and that the improved ability to select actual registered voters with some history of voting makes for a more accurate measurement of likely voters.

The shift to "registration-list based sampling" (RBS) by the Field Poll is remarkable given its history and influence among other pollsters. As every Field release reminds us, the Field Poll has "operated continuously" since Mervin Field first founded it in 1947. Field has been on of the most active and respected members of the American Association for Public Opinion Research (AAPOR), winning its award for exceptionally distinguished achievement in 1979. A shift in methodology at such a firm does not come easily.

After Mark DiCamillo, Field's current director, emailed me with a correction to the item I posted last week, I asked him about the change. Here is his response:

The Field Poll switched over to using RBS based samples last year, and used them throughout/t the 2006 gubernatorial election cycle. There are obviously pluses and minuses to the change, but long term we think this is the way to go, since the cell phone only household segment missed by RDD samples willonly get larger over time. We've found that by using the RBS samples,we can incorporate cell phone only voters since they tend to be thenewer registrants and when providing a phone contact on their voterregistration card, they provide their cell phone number. So, we've developed some special procedures for calling these listings. In addition, prior to the survey we send the entire voter sample to Survey Sampling and they identify for us which numbers on the voter list are cell phone listings, so we know in advance which numbers are cell phones. So, far we've been successful in bringing in a majority of the cell phone voters into the sample.

BTW-- Our final pre-election poll estimates in last yea's cycle were extremely accurate, in that not only did we show Schwarzenegger ahead by 16 pts (he won by 17), but our measures for each of the down ballot candidate races were all on the money as were our proposition measures, so we are encouraged by our first experiences with the change-over to these samples.

Two things are especially intriguing here: First, Field's methodology page explains that the voter list samples they purchase from the firm Voter Contact Services currently provide "a telephone number for about 85% of the voters listed." If that statistic is accurate and if the voter list is otherwise up to date, it would mean that the aggregate coverage provided by Field's RBS sample may not be that much worse than the current coverage of random digit samples nationally. According to the most recent report by the National Health Interview Survey, the proportion of U.S. households with traditional landline phone service had fallen to 87.5% as of the first six months of last year.

Similarly intriguing are the "special procedures" the Field Poll has developed to interview voters on their cell phones. Hopefully, they will have more to say about these innovations soon.


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