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Effective magnitude and effective threshold

Abstract

Effective magnitude (M') and effective threshold (T') are important because they try to express quantitatively a major aspect of electoral systems, namely the degree of squeeze they put on representation of small parties. Three relationships have previously been proposed between M' and T'. Of these, T'=75%(M'+1) is found here to have the most desirable characteristics. However, regardless of the precise equation used, a disturbing discrepancy is observed in the case of single-member districts: the effective threshold predicted is much too high, if applied nationwide. This points out a more general need to keep district-level and nationwide indicators carefully separate. An appendix proposes a new formula to find effective magnitude when district magnitudes within a country vary.

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