The unfortunate natural experiment in ballot design: the Scottish Parliamentary elections of 2007

The 2007 Scottish Parliamentary elections were notable for the extensive variation across constituencies in rejected ballots (ranging from 1.90% to 12.09%). This paper uses an unfortunate natural experiment to identify the influence of ballot design on the occurrence of rejected ballots, or 'residual votes'. In two electoral regions, visual prompts were removed and instructions were abbreviated on the (already poorly designed) ballot papers. Using zero-truncated negative binomial regression to model total residual votes as well as constituency and regional undervotes and overvotes, we find clear evidence that these changes made a major contribution to the extent of residual votes in constituencies within those regions. The findings emphasise that ballot design is not a trivial subject that can be neglected by electoral administrators.