As we pour over the latest data, it is worth taking a look back at a previous year in Iowa.
The Democratic contest in 2004 was strikingly dynamic, with a sudden surge for Kerry and Edwards as Gephardt and Dean slumped. (And note the gap in polling. Those are the holidays we are in right now when polling wasn't done.) By the last polls, Kerry had established a clear lead, but Dean, Edwards and Gephardt were within four points of each other. The polling got the trends pretty close to right. Edwards was clearly on his way up through the last 12 days of the race, as was Kerry. Dean may or may not have reversed his fall in the last week, and the polls said Gephardt was coming down. So far, so good.
But the Democratic caucus process does a lot to change the outcomes, with supporters of non-viable candidates joining forces with their second choices. That process is likely to boost the well off candidate, while robbing the struggling campaigns. And of course there is always the issue of which candidate's supporters actually turn out on caucus night.
The entrance poll is the best measure we have of whose supporters actually show up on caucus night, even with all the appropriate cautions about the entrance poll itself. Comparing the entrance poll to that last estimated trend value for candidate support from pre-caucus polling, we can see how the two rising campaigns did even better than the polling predicted. Kerry's last poll trend estimate was 25.9%, good for first place. But his entrance poll support was 34.8%. For Edwards, his final trend estimate was 21.4%, but the entrance poll found 26.2% supporting him.
For Gephardt the story was the opposite. His supporters stayed home on caucus night. The pre-caucus trend had Gephardt falling but at 17.8%. But the entrance poll found only 10.3% to be Gephardt supporters.
Dean was the only top candidate whose preelection and entrance poll numbers match closely. His trend estimate was 20.3% and the entrance poll put his support at 20.5%.
Caucus night had the effect of stretching out the differences between the candidates, advantaging the top two while damaging the fourth place candidate. (Dennis Kucinich was the exception in the back of the pack, with a trend estimate of 1.7 but an entrance support of 4%.)
Then they vote. And form coalitions with non-viable supporters. And weight the delegates in a complex formula and finally there is an allocation of delegates to the state convention. That allocation is the best we can do to consider a "final" outcome of this process.
The results there further favored Kerry and Edwards. Kerry moved up to 38% of delegates, from 34.8% in the entrance poll and 25.9% in the pre-caucus poll trend. Edwards got 32% of delegates, up from his 26.2% in the entrance poll and 21.4% in the poll trend.
Dean ended up with 18% of delegates, down a bit from the 20.5% in the entrance and 20.3% in the poll trend. And Gephardt's delegates just about matched his entrance poll, 11% of delegates and 10.3% in the entrance poll. A disappointment from his 17.8% final trend estimate.
So let's take one important lesson away from this Caucus Past. The pre-election poll trend got the order of finish right, if only by a point separating Edwards and Dean. But the process of caucus night that makes it tough to come out means that enthusiastic supporters are more likely to turn out than those who are discouraged by recent slippage. That is probably true for both parties. On the Democratic side, the complex voting and coalition formation further exaggerates the lead of the top candidates and diminishes the showing of marginal ones. And that process is seen even in comparison with the entrance polls, let alone the pre-election polling.
In Iowa the "outcome" is quite a few steps removed from the simple balance of preferences among the population. The mechanisms themselves intervene to affect the final delegate counts. So don't expect to see the pre-election polls hit the final delegate percentages very closely. If the polls get the order right, that will be good enough.
For a fair test of how good the polling is this year, wait for New Hampshire where pollsters poll, voters vote, and they just count up the results.
Cross-posted at Political Arithmetik.