A new CNN/Opinion Research Corporation poll taken 8/18-20/06 finds approval of President Bush continuing to rise, with approval now at 42% and disapproval at 57%. That is a 2 point rise in approval and a two point decline in disapproval since the last CNN/ORC poll 8/2-3. With the addition of this data point, my approval trend estimate stands at 37.2%, revised up from the previous 36.7%.
But hold on a moment. The new CNN is an outlier-- as was the AP's reading of 33% on 8/7-9, in the opposite direction.
An outlier is a poll result that is unlikely to occur, given the current estimate of the overall trend, which was 36.7% prior to the new CNN poll. That means CNN at 42% is 5.3 points higher than we would expect. Over the nearly 1200 polls taken in the Bush administration, 90% fall within -4.7 and + 4.0 percentage points of the estimated trend, with 5% of polls below -4.7 and 5% of polls above +4.0 points from the trend. At +5.3, the new CNN clearly is above the upper limit. Now that CAN happen by chance alone (and WILL, 5% of the time, by definition) and it could also signal a sudden and abrupt shift up in approval, though the lack of an obvious reason for such a shift makes me think not. What highlighting outliers does is caution us not to place too much confidence in a single outlier. Just as some Dems were delighted by the AP at 33%, now some Reps will be thrilled to see CNN "validate the President's improving ratings". Neither is likely to be supported by further polls, nor by shifts in the trend estimate. Outliers happen, but they rarely signal true change.
Based on this, I expect the approval trend is more accurately represented by the 36.7% we estimated prior to this CNN poll. A couple more polls within the expected range will bring the estimated trend back towards that level.
Even with the CNN outlier, the trend estimate remains slightly down, which is probably the more important harbinger.
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4 comments:
How does this reconcile with the Rasmussen approval rating of 43% today? I think you could make an argument that the thwarting of the London airplane bombings improved his stature with some Republicans and Independents and is starting to show up in the poll numbers. Anyway, I just found your blog and find your analysis dead on - so don't take this the wrong way. Thanks - Marc B.
In response to anonomymous comment #1, Rasmussen uses automated callers, which often produces results that are wildly at odds with human interviews. No professional campaign would use this method, even though it is substantially less expensive, because the results are unreliable.
Rasmussen is not quite comparable to the majority of surveys that use human interviewers, but a new Gallup poll carried out over the weekend and released today has Bush up 5 points to 42%, so taking this and the recent CNN polls into account, there might be some truth to the view that the thwarting of the alledged terror plot in London has improved Bush's approval somewhat --even though this bump may well prove to be short-lived.
Marc, Sage and Alexis--
I've been reluctant to include Rasmussen and SurveyUSA which use automated dialing, though based on RDD samples of phone numbers. The latter is good. The former is still unvalidated, at least in my mind. So I don't routinely include them, though I do once in a while look at them for comparisons, or in the case of SUSA's 50 state approval series which I use because there is nothing else out there that does this.
At some point when I have some time (!) I hope to do a serious comparison of their results with other polls and decide eithter to include them regularly or decide they are too discrepant from other polls to be worth including. At the moment I'm still undecided.
That said, Rasmussen does have a distinctively high "house effect", though it tracks the trend reasonably well, though perhaps not as responsive to the trend as other polls.
I'm not convinced on the plane plot explanation for CNN and now Gallup both at 42%. The other polls taken right after the plot didn't show much if any increase from those same polls' previous readings. So to see two that make a big jump but 10 days or more after the event isn't convincing to me. CNN was an outlier, and Gallup will be too. Together though they will pull the trend line up some.
The variance across polls in the last two weeks seems larger than usual. I'm not sure yet what to make of that.
I've got a full plate but will try to add a post on Gallup shortly.
Thanks, as always, for the comments.
Charles
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