A Review and Proposal for a New Measure of Poll Accuracy

Abstract

This article proposes a new measure of the predictive accuracy (A) of election polls that permits examination of both accuracy and bias, and it applies the new measure to summarize the results of a number of preelection polls. We first briefly review past measures of accuracy, then introduce the new measure. After the new measure is described, the general strategy is to apply it to three presidential elec- tions (1948, 1996, and 2000) and to compare the results derived from it to the results obtained with the Mosteller measures. Then, the new mea- sure is applied to the results of 548 state polls from gubernatorial and senatorial races in the 2002 elections to illustrate its application to a large body of preelection polls conducted in off-year races with dif- ferent outcomes. We believe that this new measure will be useful as a summary measure of accuracy in election forecasts. It is easily com- puted and summarized, and it can be used as a dependent variable in multivariate statistical analyses of the nature and extent of biases that affect election forecasts and to identify their potential sources. It is com- parable across elections with different outcomes and among polls that vary in their treatment or numbers of undecided voters.