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Using data on federal elections for both the Upper and Lower Houses in Australia and the Herfindahl–Hirschman Index (HHI), we explore whether the contestability/concentration varies across elections. Our results show significant movements in contestability over time. The results also imply a smaller degree of concentration than we would expect to find in a two-party system. To the extent that the Upper House (Senate) – until the 2004 election – is not controlled by the executive, this implies a greater level of political competition, and choice for voters, than that associated with two-party systems.
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Davidson et al. (2008) studied this question.
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