Multilevel models in meta-analysis of pre-election poll accuracy measures

Comparing the expected result of a party with the actual electoral performance, in fact, it is possible to compute a measure of poll predictive accuracy. Some scholars proposed a measure whereby the accuracy of pre-election polls is assessable. Employing such measure, we present a meta-analysis of accuracy of pre-election polls aiming to explain the differences among the accuracy measures in each poll. In such as, we intend to identify the potential sources of biases affecting poll results. We carry out a meta-analysis of accuracy measures specifying a ‘special’ multilevel regression model. The accuracy measure is used as dependent variable in a hierarchical model where each outcome is affected by a specific sampling error assumed normally distributed and with a known variance. Multilevel regression models allow including in the model the poll features as explanatory variables. The paper shows a meta-analysis of pre-election polls accuracy, carried out in parliamentary elections in Italy from 2001 to 2012.