Mark Blumenthal | December 28, 2007
Topics: 2008 , Disclosure , Iowa , Likely Voters , The 2008 Race
Here are some additional details on the new Los Angeles Times/Bloomberg poll in Iowa. The last Times/Bloomberg poll in September drew a sample of "caucus voters" that represented a much larger slice of the Iowa population than other polls. The Democratic sample represented 39% of Iowa adults, while the Republican sample represented 29% of adults. While this statistic varied greatly among pollsters, most have reported "likely caucus goer" samples representing a range of 9-17% of Iowa adults for the Democrats and 6-11% for the Republicans (see the second table in my Disclosure Project post).
For this most recent survey, the Times release did not report the percentage of adults represented by each sample, but they did provide the unweighted sample sizes for the four different Iowa subgroups they released. All four are considerably closer to the low-incidence samples reported by most of the other pollsters that have disclosed these methodological details, although even the smaller Democratic "likely caucus goer" sample (17% of adults, unweighted) appears to be on the high side of what other pollsters reported to our Disclosure Project.
I put "appears to be" in italics above because the more accurate weighted values may be different. The methodology blurb in the Times release implies that the weighted size of each sample may be slightly smaller. Though unclear on the details, they say they "designed" their sample to " yield greater numbers of voters and thus a larger pool of likely caucus goers for analysis." That design may mean that the weighted value of the caucus voter and likely caucus-goer samples may be slightly smaller. I emailed a request for the weighted values and, as of this writing, have not received a response.
Update: Just received a response and added the weighted values to the table above. The weighting does bring down the size of the two "likely caucus goer" subgroups slightly, to 15% for the Democrats and 7% for the Republicans.
Update 2: "So what does this mean?" Two commenters ask that question, so I obviously neglected to explain. For those interested in all the details, the complete context can be found in this section of my Disclosure Project results post. The key issue is that the previous historical highs for caucus turnout are 5.5% of adults for the Democrats in 2004 and 5.3% of adults for the Republicans in 1988. Pollsters are generally not trying to screen all the way down to a combined 11% of adults, since (a) no one knows what turnout will be next week, (b) low incidence screens cannot select truly "likely" caucus goers with precision and (c) all political surveys presumably have some non-response bias toward voters (on the theory that non-voters are less interested and are more likely to hang up).
On the other hand, I consider it highly questionable to report results representing 68% of adults as representative of "caucus voters" as the Times/Bloomberg survey did in September.
So the results above mean two things. First, the latest Times/Bloomberg surveys are a vast improvement in terms of the portion of Iowa adults they represent. Second, at least in theory, the "likely caucus goers" are the more appropriate subgroups to watch. Of course, the percentage of adults sampled is just one aspect of accurately modeling the likely electorate. The kinds of voters selected are just as important, and can vary widely across polls that screen to the same percentage of adults. See the full Disclosure project post for more details.