Obama Approval Index Rating = 0

As we have noted many times, there is a disagreement within the polling industry as to whether or not polling firms should “weight” or adjust their sample to reflect a specific mix of Democrats, Republicans, and unaffiliated voters (see recent article on why some polls produce different results).

Rasmussen Reports does weight our sample to a set of partisan targets and bases those targets on surveys conducted in preceding months. Entering the month of June, our targets for the month were set so that the sample would include 9.44% more Democrats than Republicans. At the end of the month, a review of the data showed that if we had not used the party weights, the Democrats would have had a 9.37% advantage over the GOP. The bottom line is that—over the course of a full month—our results would have been the same with or without party weighting.

However, the value of party weighting for a tracking poll becomes clear when you look at the numbers on a day-to-day basis.

The table of data for June highlights the percentage of Republicans and Democrats in our sample each day in June. These numbers are derived after we weight for all factors except party and they 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 fluctuations reported would have been nothing more than statistical noise. There has been little or no movement in the race for the White House since Barack Obama clinched the Democratic Presidential Nomination. Including leaners, Obama has been at 48% or 49% support in our daily Presidential Tracking Poll for twenty-one straight days leading up to the Fourth of July weekend. McCain has generally been at 43% or 44% during that time from. The full month results show Obama leading McCain 49% to 44%.