Oct. 24: In Polls, Romney’s Momentum Seems to Have Stopped

By NATE SILVER

The term “momentum” is used very often in political coverage — but reporters and analysts seldom pause to consider what it means.

Let me tell you what I think it ought to mean: that a body in motion tends to stay in motion. That is, it ought to imply that a candidate is gaining ground in the race — and, furthermore, that he is likely to continue to gain ground.

As a thesis or prediction about how polls behave, this notion is a bit dubious, especially in general elections. In races for the United States Senate, for instance, my research suggests that a candidate who gains ground in the polls in one month (say, from August to September) is no more likely to do so during the next one (from September to October). If anything, the candidate who gains ground in the polls in one month may be more likely to lose ground the next time around.

(More here.)