The importance of measurement error in polls: Dissecting the latest Washington Post-ABC News poll result on swing states

With a margin of error either way, an 11-point spread isn't anywhere near as big as people might think.

You’ve got to get deep into the weeds to tell what is going on. The Washington Post-ABC pollsters tell us that “52 percent of likely voters across swing states side with Obama and 41 percent with Romney in the new national poll.” But without the proper context, readers may jump to an incorrect conclusion when they see that figure, concluding that Obama is home-free in swing states. As I learned from Post pollster Jon Cohen, that finding is based on the responses of a total of 160 people, and it has a margin of error of 8 percentage points. So yes, there may be a difference between swing-state and national numbers, but the gap might be very small or it might be big.