Des Moines Register poll shows Trump and Clinton leading

The final Des Moines Register poll, released over the weekend, shows Donald Trump leading the Republican field in Iowa. He has 28 percent support compared to 23 percent for Ted Cruz and 15 percent for Marco Rubio.

Among Democrats, Hillary Clinton leads Bernie Sanders by a narrow 45-42 tally.

If this poll accurately predicts the outcome, it won’t be a happy night for me. I’m rooting for Sanders and either Cruz or Rubio.

How good of a predictor, though, has the final Des Moines Register poll been? Quite good, according to Henry Enten of FiveThirtyEight. He writes:

History suggests they’re a good indicator of what will happen in Monday’s Iowa caucuses, though there is room for a candidate or two to surprise. I’ve gathered [the pollster’s] final caucus polls for Republicans and Democrats since she started at the Register [with the 1988 caucus]. For each race, I’ve calculated the difference between all candidates’ polled percentage of support and their final share of the vote.

Selzer’s final poll has correctly projected every single winner except for Republican Rick Santorum in 2012.

And the Register poll’s successes haven’t been limited to blowout races. She caught the late momentum for Democrat John Kerry in 2004 and famously called Barack Obama’s victory in 2008.

Even when [her] final poll missed the winner in 2012, it still indicated the potential for Santorum to win. The survey was conducted over four days, and in addition to the topline, aggregate numbers, [she] released day-by-day results. The final day of her last 2012 caucus poll showed Santorum within a percentage point of Mitt Romney. In fact, the average error per candidate of the last day of her final 2012 caucus poll was just 1.9 percentage points.

According to Enten the Register isn’t just good at picking winners, it also does a good job predicting vote share for all candidates:

[The] average error per candidate per year has been just 3.3 percentage points. That means that what a candidate receives in [the] poll is probably going to be pretty close to what he or she gets from voters.

Fortunately, the Des Moines Register occasionally misses big on a particular candidate. Moreover, the leads of Clinton and Trump aren’t large.

So what type of candidate is most likely to outperform his or her showing in the poll?

History suggests there are two types. . .The first is a candidate who does well among Christian conservatives. [The] final polls on the Republican side in 1988, 1996 and 2012 all missed the candidate favored by Christian conservatives by at least 8.5 percentage points. That could be good news for Cruz.

Secondly, candidates with late momentum, such as Kerry in 2004 and Santorum in 2012, also tend to beat their polls. That could be beneficial to Rubio, who seems to be gaining in some polls.

For better or for worse, all will be revealed very soon.

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