Abramowitz: A Note on the Rasmussen Effect
This item by Alan Abramowitz, Alben W. Barkley Professor of Political Science at Emory University, and a member of TDS' Board of Advisors, is cross-posted from Pollster.com.
In his recent post, Mark Blumenthal provides an excellent discussion of some of the possible explanations for the differences between the results of Rasmussen polls and the results of other national polls regarding President Obama's approval rating. What needs to be emphasized, however, is that regardless of the explanation for these differences, whether they stem from Rasmussen's use of a likely voter sample, their use of four response options instead of the usual two, or their IVR methodology, the frequency of their polling on this question means that Rasmussen's results have a very disproportionate impact on the overall polling average on the presidential approval question. As of this writing (December 4th), the overall average for net presidential approval (approval - disapproval) on pollster.com is +0.7%. The average without Rasmussen is +7.1%. No other polling organization has nearly this large an impact on the overall average.
A similar impact is seen on the generic ballot question reflecting, again, both the divergence between Rasmussen's results and those of other polls and the frequency of Rasmussen's polling on this question. The overall average Democratic lead on pollster.com is 0.7%. However, with Rasmussen removed that lead jumps to 6.7%. Again, no other polling organization has this large an impact on the overall average.
According to Rasmussen, Republicans currently enjoy a 7 point lead on the generic ballot question among likely voters. Democracy Corps, the only other polling organization currently using a likely voter sample, gives Democrats a 2 point lead on this question. To underscore the significance of this difference, an analysis of the relationship between popular vote share and seat share in the House of Representatives indicates that a 7 point Republican margin of victory in the national popular vote next November would result in a GOP pickup of 62 seats in the House, giving them a majority of 239 to 196 over the Democrats in the new Congress. This would represent an even more dramatic shift in power than the 1994 midterm election that brought Republicans back to power in Congress. In contrast, a 2 point Democratic margin in the national popular vote would be expected to produce a GOP pickup of only 24 seats, leaving Democrats with a comfortable 234 to 201 seat majority.
One of the biggest problems in trying to compare Rasmussen's results with those of most other polls is that Rasmussen is almost alone in using a likely voter sample to measure both presidential approval and the generic ballot. Moreover, Rasmussen has been less than totally open about their method of identifying likely voters at this early stage of the 2010 campaign, making any evaluation of their results even more difficult. However, there is one question on which a more direct comparison of Rasmussen's results with those of other national polls is possible--party identification. Although the way Rasmussen asks the party identification question is somewhat different, reflecting its IVR methodology, Rasmussen's party identification results, like almost all other national polls, are based on a sample of adult citizens. Despite this fact, in recent months Rasmussen's results have diverged rather dramatically from those of most other national polls by showing a substantially smaller Democratic advantage in party identification. For example, for the month of November, Rasmussen reported a Democratic advantage of only 3 percentage points compared with an average for all other national polls of almost 11 percentage points.
Rasmussen's party identification results have only a small impact on the overall average on this question because they only report party identification once a month. However, Rasmussen's disproportionately Republican adult sample does raise questions about many of their other results, including those using likely voter samples, because the likely voters are a subsample of the initial adult sample. If Rasmussen is starting off with a disproportionately Republican sample of adult citizens, then their likely voter sample is almost certain to also include a disproportionate share of Republican identifiers. Of course, there is no way of knowing for certain whether Rasmussen's results are more or less accurate than those of other polling organizations. All we can say with some confidence is that their results are different and that this difference is not just attributable to their use of a likely voter sample.