A moving target? An analysis of the impact of electoral context on polling error variation in both British and international general elections

Abstract

ABSTRACT In this article, we argue that electoral context affects the projection mechanisms inherent within polling. This insight applies both to the estimation of party vote shares by pollsters and to poll-based seat projections. To test this argument, we analyse 794 in-campaign polls covering the UK's 21 post-war general elections, as well as an updated version of Jennings and Wlezien’s (Jennings, Will, and Christopher Wlezien. 2018. “Election Polling Errors Across Time and Space.” Nature Human Behaviour 2 (4): 276–283) international polling dataset. We demonstrate that the election level houses a substantial portion of the observed variance within polling error. This finding is valid across several modelling approaches and a range of measures of polling accuracy both within and beyond the UK. Within the UK, we show that the election level is a particularly important locus of variance when it comes to analysing whether polls give rise to misleading expectations concerning seat distributions.