Campaigns, Polls, and the States: Assessing the Accuracy of Statewide Presidential Trial-Heat Polls

Given the heavy reliance upon polls during election campaigns and the importance of state results in presidential election outcomes, this study examines the determinants of accuracy in statewide presidential trial-heat polls. Using Lau's (1994) examination of national-level polls as a guide, we find that sample-related characteristics like sample size and the use of a likely voter sampling filter tend to improve the predictive accuracy of these polls. More importantly, however, we find that the number of days to election has an important impact on poll accuracy and that this effect can vary substantially across different campaign contexts. Preference polls have long been a staple of media coverage of election campaigns. Not a day goes by during the height of a presidential campaign, it seems, that one doesn't come across some results of a particular poll, indicating who's up and who's down. Indeed, pundits often go to great lengths to explain modest movements in the polls-whose particular strategy seems to be working and whose seems to be falling flat. However, the art of interpreting polls often appears to overshadow the science of administering and applying them. Nowhere is that perhaps more apparent than in the duality with which candidates themselves appear to treat them. Outwardly, candidates often make it a point to downplay the meaning of the polls, whether they are leading or behind. The front-runner typically fears overconfidence and complacency among the ranks, while the trailing candidate is concerned about maintaining morale. Behind the scenes, the candidates' campaigns are closely monitoring the polls and adjusting the message accordingly (Jacobs and Shapiro 1994). Political scientists studying voting and elections have a particular interest in the science side of such polls because they provide an opportunity to test specific hypotheses and, ultimately, attempt to predict outcomes. As these polls become more widely used by media outlets, the data have become more accessible to, and are increasingly used by, political scientists and other potential consumers. For example, aggregate national poll results have been used in political science research to assess campaign effects (Hoibrook 1996), evaluate the sources of error in polls (Lau 1994), and ultimately, forecast outcomes (Campbell 1996). It has only been recently, however, that poll data from the 50 states have become more widely available. It would seem that the results of state polls could also serve many useful functions in the areas of public opinion and voting behavior research. First, the use of state polls complements the state-centered nature of our presidential selection process. The electoral college effectively turns each presidential election into 50 separate state contests (plus the District of Columbia). Second, the use of state rather than national polls provides many more observations, and consequently more variation, both in terms of the phenomenon being measured and the methods used to measure it, that can be incorporated into analyses. This study attempts to take advantage of these benefits by utilizing data from statewide "trial heat" polls conducted during the fall campaigns of the 1992, 1996, and 2000 presidential elections. Our task will be to assess the general accuracy of these polls and examine the possible sources of error in their results. In other words, we will be assessing how well these polls did in describing the actual outcomes, and what, if any, specific factors can be identified to account for variation across polls in their level of accuracy. THE DETERMINANTS OF ACCURACY IN STATEWIDE TRIAE HEAT POLLS Lau (1994) offers a systematic glimpse of the factors that contribute to the accuracy of trial heat polls conducted at the national level. Specifically, his study of 56 national trial heat polls taken during the 1992 Presidential election campaign indicates that factors such as number of days over which the poll was conducted and the use of a tracking poll improved the accuracy of the polls. …