An analysis of the accuracy of "trial heat" polls during the 1992 presidential election

There is little systematic research on the multitude of factors that influence the accuracy of poll results. This article examines six methodological factors directly under the survey researcher's control and two exogenous factors concerning the nature of public opinion as sources of survey error. Data for this study come from 56 "trial heat" polls conducted during the last month of the 1992 presidential election. The most important variables influencing survey accuracy were the number of days a poll is in the field, which increased total accuracy one-half of a percentage point per day; conducting interviews only on weekdays (and thus only during evening hours), which reduced overall accuracy rates by more than 1 percentage point; and conducting a "tracking" poll, which increased accuracy by about 1.5 points. Sample size was not related to accuracy rates. Results also indicated that sampling frames of "likely voters" (relative to "registered voters") tended to overestimate support for George Bush and underestimate support for Ross Perot, that interviewing only on weekdays led to overestimates of support for Bush, and that strict methods of defining a respondent as "supporting" a candidate hurt the two newcomers, Perot and Bill Clinton, more than Bush. In light of these data it is recommended that the common practice of reporting "margins of error" based solely on sample sizes be abandoned as misleading and replaced by a more empirically justifiable measure based more on response rates.