TitleUnderstanding Party Vote Share and Split Voting: An Application of Bland-Altman Difference Plot and Political Relative Development Index
作者Shun-chuan Chang, Wen-jong Juang, Cheng-hsiang Chang

Assistant Professor, Holistic Education Center, Mackay Medical College
Associate Professor and Chair, Department of Public Policy and Management, Shin Hsin University
Master of Department of Public Policy and Management, Shin Hsin University

AbstractTaiwan has been under electoral reforms in recent years: Legislator election in 2008 started to adopt “single-district two votes system”. Combined legislative and presidential elections in 2012 allowed voters to vote for president, at-large legislators and local legislators concurrently for the first time, offering a great opportunity to study party vote share and split voting in general elections. There were practical researches on split voting, some of which analyse micro information from polls, and some are based on the macro data of votes in elections. However, these studies took into consideration neither the size of eligible voters in districts nor the ratio of national votes to regional votes a party or candidate get, which thus makes it hard to determine the correlation between split voting, the size of electoral districts and a party’s real political power strength.
This study attempts to examine party vote share and split voting from a novel perspective on proposing a new measurement and exemplifying with the general election in Taiwan in 2012. The research started with calculating the vote share of the Pan-Blue Coalition, the Pan-Green Coalition and each party in 368 administrative districts and gauging the voting gap with a traditional inspection method. Secondly, we tried to use a Bland-Altman difference plot to show the pattern of split voting by districts since every party’s ability to gain votes varies with areas. Thirdly, we discussed the strengths and inadequacies of the application to Bland-Altman difference. And then, the political relative development index, BDI and CDI, are introduced as the new framework for ascertaining party vote share and measuring split voting to decide the relative level of voters’ support for some party, the Pan-Blue Coalition or the Pan-Green Coalition. Finally, the result from the new measurement is compared with that from the traditional method to validate the performance of the new approach, as well as to indicate future research direction.

Keywords: split-ticket voting, Bland-Altman difference plot, political relative development index, party vote share, election study
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