Polling the Iowa Caucuses 

 The presidential caucuses in Iowa will be held tonight, giving us our first "official" measure of popular support for the candidates in each party.  We've had lots of "unofficial" measures from polls taken over the past few months - over 50 polls taken since Labor Day are posted on pollster.com - but polling to predict the outcome of the caucuses (as opposed to polling designed to measure overall support for the candidates) presents a number of difficult problems. 

 The first of these problems, and the one that has received the most attention, is identifying likely caucus participants from the sample of respondents.  The Iowa caucuses require a significant time commitment (on the order of two to three hours) in order to participate, and turnout has historically been much lower than it has in the New Hampshire primary, to say nothing of a general elections.  Identifying likely voters is a key challenge for any poll, but the low turnout levels make survey results unusually sensitive to the screening assumptions used.  The recent Des Moines Register poll showing Obama at 32% ahead of Clinton at 25% prompted a great deal of discussion on this topic.  These estimates were produced from a screen that implied that 60% of participants tonight would be first time caucus-goers and only 54% would be registered as Democrats before the caucus.  While these could be valid estimates (we will soon see), this would be a very different population of caucus participants than we have seen in the past.  For more discussion of the screening issue, see  this post  at  Mystery Pollster , which is one of the best sites for coverage of the various polling-related issues in the current campaign.   
 


 On the Republican side, identifying likely caucus-goers is the main methodological problem.  Iowa Republicans basically take a straw poll and report the results, so a random sample of participants (if they could be identified without error and did not change their minds) would produce an unbiased estimate of the outcome.  Things are much more complicated on the Democratic side (their caucus guide is thirteen pages long); caucus participants first break into preference groups for each candidate.  After this is accomplished, there is the opportunity for realignment subject to a "viability threshold" which varies from precinct to precinct but is always at least 15%.  Pollsters may attempt to model this by asking respondents for their second choice and reallocating those respondents supporting candidates estimated to be below the threshold on a statewide basis.  This, of course, assumes a uniform distribution of support across the state, which may or may not be a reasonable assumption (or may be more reasonable for some candidates than for others).  If support for Kucinich, hypothetically, is concentrated in Ames and Iowa City, then he may be viable in the precincts where most of his support is found despite being well below the threshold statewide.   

 Finally, the Democrats in Iowa do not act or report results on a "one-person-one-vote" basis.  The precinct caucuses elect delegates pledged to each of the candidates, and the number of delegates is based on the historical support for Democratic candidates in that precinct, not the number of people who participate in the caucuses.  The state party then takes these results and calculates the "state delegate equivalent" share.  The raw vote totals are available to the state party (which would be the closest to the parameter that the pre-caucus polling is trying to estimate) but the party does not release those results to the media (an  op-ed criticizing this practice  appeared in the New York Times last month).  To my knowledge, none of the groups polling in Iowa attempts to take this weighting into account.  The degree to which this causes the reported results to diverge from the raw votes will depend largely on the degree to which turnout in the caucuses diverges from historical Democratic turnout; if the Register poll is correct, this divergence could be quite large (and Obama supporters might not be too happy). 

 To sum up, polling the Iowa caucuses in order to predict the outcome for the Democrats is a serious problem: the population is hard to define, preferences are likely to be unusually malleable since the party rules require some participants to change their votes, and the results that are reported are not the quantity that would be estimated by a simple random sample of participants.  If the polls appear to have gotten it wrong, it will be hard to parse out which of these factors (in addition to the normal sources of bias in any survey) were the main contributors.