Response from Sam Wang,
September 17, 2014 1:19pm
First,
a small correction. In the FiveThirtyEight article,
there may be confusion between snapshots and predictions. The Princeton
Election Consortium provides snapshots as a precise measure of where the race
stands on any given day. In contrast, predictions are less certain. This is
why, for the question of whether Democrats+Independents
reach 50 votes, the snapshot is at 80%, and the prediction for Election Day is
at 70%. As a validation of PEC's this poll-based predictive approach: when
applied to Presidential races, it gives a cliffhanger in summer of 2004 and
likely Obama wins in summer of 2008 and 2012.
The
current argument distracts from the real point: PEC is doing a polls-only
calculation, as do HuffPost, Electoral-Vote.com, RealClearPolitics, DailyKos Poll
Explorer, and TalkingPointsMemo. All of these show a
tight race, and in some cases show Democrats+Independents
slightly favored. If one adds assumptions about where the Senate race
"ought" to be, the outcome looks a little bit different. Several
major models (NYT, 538) have said that at the start of
2014, conditions favored the GOP. However, for most of the year,
polls have shown that Republicans are slightly underperforming, relative to
those expectations. That is the real political story.
In
my view, it is helpful for readers to be able to separate what the polls say,
and where data pundits say the polls ought to be ("fundamentals").
The NYT's Upshot does that here - it's a good explorational
tool for anyone who wants to understand this important disagreement.
http://www.nytimes.com/interactive/2014/upshot/elections-2014-make-your-own-senate-forecast.html
Finally,
regarding Senate polls: in the aggregate, they are pretty good. When poll
medians are applied to Senate races, they perform slightly better than
polls-plus-fundamentals. In 2010, both approaches worked - though they
both got the Nevada Senate race wrong. And then, of course, in 2012, there were
two differences:
PEC: http://election.princeton.edu/2012/11/06/senate-prediction-final-election-eve/
538: http://fivethirtyeight.blogs.nytimes.com/fivethirtyeights-2012-forecast/