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Republican Bounce and Trend Sensitivity


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The post-convention bounce is now moving in the Republican direction, but with an enormous spread in estimates. A Gallup/USA Today (9/5-7) has an enormous 10 point McCain lead over Obama, 54%-44%. In contrast, Gallup's tracker over the same days shows a 5 point McCain lead, 49%-44%. Now would be a good time to note that the tracker is a registered voter (RV) sample, while the Gallup/USAT is a likely voter (LV) sample. LV samples typically are more favorable for Republican candidates, so at least some of this difference is probably due to these different sampling frames. We'll no doubt be talking a lot about this issue in days ahead.

But other polls on the same days show a tied race. Diageo/Hotline has the race 44%-44% and CNN has it 48%-48%. And Zogby's Internet poll done 9/5-6 puts the race at 50%-46%.

All of these are much better for McCain than the 5-9 point Obama leads we saw in the immediate aftermath of the Democratic convention.

So it looks like both parties got nice convention bounces.

Our trend estimate is still hungry for more data. The standard, blue line, estimate is now less persuaded that Obama had a convention bounce OR that McCain is getting one either. That is standard behavior of our estimator which is designed to be a bit conservative when faced with conflicting polls and short term changes of trend.

But that is why we have our "sensitive" estimator for comparison. The red line is a trend estimate that is about twice as sensitive a the blue line. It is considerably quicker to respond to short term changes and to fewer polls. The down side is it will often chase random noise.

Since there is good reason to believe convention bounces are real, it is reasonable to think that the red line's indication that the race has indeed tightened is probably a real signal in the data, and not just noise. On the other hand, the Gallup 10 point McCain lead is out of the range of any other current polling data. So "red" may be chasing that outlier just a bit more than is good for him. As the figure makes clear, red and blue usually agree quite closely after enough data are in hand, but can diverge especially when data are sparce.

A prudent approach is to wait for a few more post-convention and post-weekend interviewing polls to see just how big and how sustained the RNC bounce is. But both estimators agree we have ourselves a real horserace now.

Cross-posted at Political Arithmetik.

 

Comments

The voice of sanity weighs in. Thank you, Mr. Franklin :-)

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I found this comment particularly interesting in today's Rasmussen daily report:

For a variety of reasons, the Rasmussen Reports daily Presidential Tracking Poll is less volatile than some other polls and always shows a somewhat smaller convention bounce than reported by others. This is primarily because we weight our results by party identification.

I wonder, then, how much of the movement in Gallup is due to excitement among Republicans that is not being captured in Rasmussen because they are weighting down Republican identifiers.

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dave101:

Party identification has never meant less, relatively speaking. More people then ever will decide based on candidates, rather than party. (I just read this but can't remember the source - NYTs?)

This would mean Rasmussen poll is less meaningful.

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stingray1965:

It's funny...I always hoped that we Democrats were a stronger-hearted lot. I have never seen so much handwringing in all my life. We got our shot during the DNC and went up a few points...they got their shot and now they are up a few points. We have four debates ahead of us...these two men have yet to be seen on the same stage debating the issues. It's time for my brother and sister Dems to quit whining and get to work.

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Basil:

@stingray

You said it , brother. I see this as a case where voters are like a jury who believe the last thing they heard before rendering a verdict. Now, that would be the R convention and the Palin nomination. It's the advantage of going last. Fortunately, the election isn't today.

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Are these trend estimators mean-based? It would be interesting to employ some form of outlier rejection. For instance, to weight all polls equally one could use some form of the median, which is what I do. However, it's not clear to me how to do that for a more sophisticated tracker like the one used here.

Sam Wang
Princeton Election Consortium

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hyh:

A thought...

Given the short amount of time between this year's two conventions, measuring how big the bounce was from the second convention might be more complicated.

One could argue that the second bounce should be measured from when the convention started. By this method, McCain had a bigger bounce as everyone has been saying.

However, another method would be to use the polling data from 2-3 weeks after the second convention as the baseline to measure how big the second bounce was. Perhaps this is something to watch out for?

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