There is a bit of a controversy swirling over a statement made by economist and columnist Paul Krugman. He writes:

"But anyone who takes the trouble to look at the numbers knows that the thrill is gone. Mr. Bush's approval ratings have plunged over the last two months. A year ago he was, indeed, immensely popular; right now he's not significantly more popular than he was before Sept. 11.

The famous columnist Andrew Sullivan in his website sharply criticizes Krugman. His basic point:

“But if you average it all out, the drop is probably around 3 - 4 percent from low 60s to high 50s - still in the region of an electoral landslide, and in line with the months before. The dictionary definition of a "plunge" is "to descend steeply; fall precipitously; to move forward and downward violently; to become suddenly lower; decrease dramatically." The column headline? "Credibility Problems." Yep. He got that right. (With thanks to Don Luskin).””

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Let’s see what the numbers have to say. We plot the results of the ABC news poll of likely voters over since 9/11/2002:

Bush's Approval Rating By Date of Poll

There is no statistical definition of plunge,but it sure looks like a precipitous drop to me. Wait! I encourage the reader before flaming back at Sullivan to read further. Look at the following graph:

Bush's Approval Rating By Date of Poll

Here we plot the entire time series starting from 9/11/2001. The conclusion we make here is very different. There is no plunge- only the steady continuation of a linear downward trend. Approval ratings going down? Absolutely. Plunging? NO. At least not with respect this time scale.

Now that we have established a steady trend of a couple of point down a month, we revist the changes after subtracting out the trend. This produces a residual plot of the same data:

Now we discover the problem. With respect to the post 9/11 trend, there is one unusual point (marked with a +: 12/31/2002). This is the largest residual value in the data.

Conclusion:

Bush’s approval has been declining at a STEADY pace since 9/11/2001. At this point his rating have returned to his pre-September 11 numbers. At the end of 2002, his numbers stopped declining, which gives the appearance of a plunge at the end of January, 2003, since those numbers are in line with the overall trend.