It’s probably about that time in the election cycle to repeat some analysis offered by Nate Silver in 2010 that remains highly relevant today: “momentum” in politics is a vastly overused and sometimes downright erroneous concept:
Turn on the news or read through much of the analysis put out by some of our friends, and you’re likely to hear a lot of talk about “momentum”: the term is used about 60 times per day by major media outlets in conjunction with articles about polling.
When people say a particular candidate has momentum, what they are implying is that present trends are likely to perpetuate themselves into the future. Say, for instance, that a candidate trailed by 10 points in a poll three weeks ago — and now a new poll comes out showing the candidate down by just 5 points. It will frequently be said that this candidate “has the momentum”, “is gaining ground,” “is closing his deficit,” or something similar.
Each of these phrases are in the present tense. They create the impression that — if the candidate has gone from being 10 points down to 5 points down, then by next week, he’ll have closed his deficit further: perhaps he’ll even be ahead!
There’s just one problem with this. It has no particular tendency toward being true.
Nate went on to look at his database of polls dating back to 1998, and concluded there was no predictive value at all in poll improvements by a candidate.
Although this analysis might seem modestly technical, the conclusion is really pretty simple. In general elections, the direction in which polls have moved is not predictive of the direction in which they will move.
Thus, it is usually wrong to say that a candidate is gaining ground in the polls — present tense — or that her position is improving. Instead, you should say that the candidate has gained ground or that her position has improved.
Thus, for example, Joni Ernst’s six point lead in the new Iowa Poll should be alarming to Democrats not because it’s an indicator she’ll be leading by eight in two weeks: it’s a present indicator that she has a sizable lead in a fairly static race that most had thought (and some still do) tied. It’s an important distinction to keep in mind as more information pours in between now and November 4.