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Mark Blumenthal

Mark Blumenthal

Posted: July 7, 2008 12:17 PM

Rasmussen's Unweighted Party ID

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Yesterday, Rasmussen Reports did something a little unusual: They published what their party identification results would have been for every release in June had then not weighted by party. The results, according to their report,

show the expected statistical noise and results that bounce around generally within the margin of sampling error. The percentage of Republicans was within the margin of error from the full month sample on 29 of 30 days. For Democrats, there were four days where the daily results departed from the full month average by more than the theoretical daily margin of sampling error. But, in all cases, even those variances were quite modest.

Even these modest variations on a daily basis produced some significant differences in terms of the gap between Democrats and Republicans. On June 11, the gap was just 3.01 percentage points. On June 18, it was 16.4 percentage points.

To understand how this might impact a tracking poll, look at the final column in the table which shows a three-day rolling average of the gap between the parties. On days when the Democratic advantage is a bit larger, we would have showed a bigger lead for Obama. When it’s smaller, we would have shown McCain gaining ground. Pundits and bloggers would have tried to explain the bouncing by whatever the candidates had said or done in recent days even though most voters are not that closely attuned to the daily rhetoric.

In reality, of course, nothing happened.

The write-up touches on Rasmussen's use of "dynamic weighting," though it leaves me a slightly confused on one minor point. Two years ago, when Rasmussen announced their procedure, they explained that weighting would be based on targets from "results obtained during the previous three months" of interviews. They routinely publish their monthly party ID numbers. The results from June mentioned in the above report, showing Democrats with "a 9.37% advantage over the GOP," are slightly different from the 9.5% advantage in their latest "Summary of Party Affiliation" table. Obviously, the difference is tiny -- perhaps one number is based on "likely" voters and the other on adults or registered voters?

One question all of this raises -- also asked by reader jsh1120 over the weekend -- is about the procedure Rasmussen uses for weighting their state level polls: Is it the same? And if so, how many interviews have they done over the last three months in each state as a basis for their party targets?

Update: Scott Rasmussen emails with an explanation of how they arrive at party weighting targets at the state level.

 

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