Tuesday, October 30, 2007

Bush Approval: Trend at 32.6%

There have been several polls since my last update, and they have varied much more than usual. But the bottom line, with all the data, is a small decline in the trend estimate, and that well within the uncertainty of the trend.

The newly added polls are:

Fox: 10/23-24/07, Approve 35%, Disapprove 55%
LATimes/Bloomberg: 10/19-22/07, Approve 35%, Disapprove 60%
ARG: 10/18-21/07, Approve 25%, Disapprove 67%
CBS News: 10/12-16/07, Approve: 30, Disapprove 61%
CNN/ORC: 10/12-14/07, Approve 36%, Disapprove 61%
Zogby/Reuters: 10/10-14/07, Approve 25%, Disapprove 75%

That is quite a range from two at 25% to a top of 36%. Zogby and ARG are well below the trend estimate, while the two at 35% and one at 36% are only a bit above trend. (An older NPR poll at 38% is well above expectations, but as I explained in an earlier post this has something to do with the likely voter sample NPR favors, compared to the adult samples of most approval polls.)

If we look at the residuals, NPR looks like a high outlier and Harris a low one. But Zogby/Reuters and ARG are even more extreme low outliers. Zogby uses the same 4 point job rating measure that Harris uses, so that partially explains their low value (a persistent question wording effect) but as the plot makes clear, the Zogby reading is even lower than we'd expect from that. The ARG reading is also extremely low, at the same 25% approval rating, and the points and labels are overwritten by each other.

The curiosity is why two (three with Harris) outliers? Did opinion change and these caught it early? Apparently not, judging by other recent polls that are at or even a bit higher than the trend estimate of 32.6%.

In general I think it is a bad idea to seek a substantive explanation for outliers. The most reasonable story is simply "random variation" and we should leave it at that. There ARE some systematic elements, such as the question wording variation or sampling issues I pointed out, but I'm not inclined to say more. To do so becomes a post hoc search for what are most likely statistical phantoms. (Though when history keeps repeating itself we might look into house effects for a systematic effect, possibly due to question wording, sampling, or treatment of don't know responses.)

The bottom line is approval may have shifted down a tad, from 33.0% to 32.6%. BUT, one should consider the gray region around the trend line below. This gives you a good idea of the uncertainty in the trend estimate itself, after squeezing out as much random variation in the polls as possible (at least until next week! Stay tuned for that!). Clearly the change of .4 percentage points is not a clear indication of movement in approval. In fact, given the wide range of current polling, we have an unusually wide uncertainty about where approval actually is at the moment, with 32.6% being our best estimate, but an uncertain one.

We are in a period of relative stability in President Bush's approval rating but considerable polling variation. Waiting for the next "thing" to happen.