Conclusions: Party-List CHPV 2

Party-List CHPV versus the Single Transferable Vote

The electoral method for both party-list CHPV and Single Transferable Vote (STV) multiple-winner elections involves organising the voting into multiple local-area few-winner constituencies. Typically, they both have about five seats per constituency. For STV, having a huge number of candidates on a ranked ballot would be impractical for voters and election staff to handle; hence the few-winner restriction. For CHPV, having just a few seats is necessary to maintain overall party proportionality.

With party-list CHPV and STV, every elected member has exactly the same status as every other member. Therefore, they all have the same set of local responsibilities and are all accountable to a specific subset of the whole electorate. This avoids having two different types of member that mixed member systems typically generate. As both systems occupy the same electoral niche, a direct comparison between their relative merits and drawbacks is instructive.

The main difference between the two systems is that in STV voters rank the candidates according to their own preferences while in CHPV they just vote for their preferred party and its closed party list. This difference results from two diverse algorithms for translating voter preferences into seats. The complex non-deterministic STV algorithm is difficult for voters to comprehend and counting is often very lengthy due to the number of candidates and rounds involved. In contrast, the party-list CHPV algorithm is simple, transparent and deterministic and the count process is quick and efficient.

To compete directly with STV in terms of expressing preferences for candidates, voters in a (nominally closed) party-list CHPV election may additionally be allowed to rank the candidates of their chosen party using a free list. A closed list is assumed solely for the purpose of allocating a proportional share of the seats to each party according to its share of the vote. Ranked ballot CHPV may then be used to determine which of the candidates on a free (unordered) party list are selected to fill the seats won by that party.

Hence, in both systems, parties nominate their own choice of candidates but it is only voters that rank them. It should be emphasised though, that unlike STV, voters using CHPV cannot affect the ranking of candidates outside their chosen party. The length of a ranked ballot is therefore much shorter with CHPV as there is a separate one for each party and supporters only compete the ballot for their chosen party.

For a fair comparison of the party proportionality of STV to that of CHPV, it has to be assumed that all voters vote strictly on party lines for a given slate or list of ranked party candidates. On this basis, STV is equivalent to the largest remainder party-list method that employs the same quota; such as the Hare or Droop Quota. Therefore, the trade-off here between susceptibility to party cloning and party proportionality (together with any associated systemic bias) is determined by the choice of this quota.

Party-list CHPV is as likely as the Droop Quota method to produce an optimally proportional outcome in few-winner few-party elections. The Hare Quota is an optimally proportional voting system but it is much more vulnerable to party cloning than is CHPV. As an equivalent largest remainder method here, STV satisfies the quota rule and so consequently suffers from certain voting paradoxes. In contrast, as party-list CHPV is a highest averages method, it is monotonic and therefore not prone to such paradoxes.

Summary Conclusion

Party-list CHPV is a simple yet reliable, practical and party-proportional voting system for use in multiple-winner elections partitioned into concurrent closed-list contests across numerous local few-winner (W ≤ 6) constituencies where invulnerability to cloning by parties with minority support is required. Additionally, supporters of a party may be permitted a free party-list in the form of a CHPV ranked ballot of their party candidates in order to select the winners of the seats won by their party.

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