Proportionality for solid coalitions

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Proportionality for solid coalitions (PSC) is a fairness criterion for ranked voting systems. It is an adaptation of the quota rule to voting systems in which there are no parties, the voters can vote directly for candidates, and voters rank individual candidates. The criterion was first proposed by the British philosopher and logician Michael Dummett.^[1][2]

PSC is a weak definition of proportionality, which only guarantees proportional representation for cloned candidates, also called solid coalitions. In other words, voters must rank all candidates within the same party first (before candidates of other parties) to ensure it is adequately represented. On the other hand, PSC does not guarantee proportional representation if voters rank candidates of different parties together (as they will no longer form a solid coalition). As a result, PSC systems like the single transferable vote^[3] can become disproportional if there are substantial cross-cutting cleavages.^[4][5][6]