Bavarian MMP

From electowiki

"Bavarian MMP" (Bavarian Mixed-Member Proportional voting) describes the procedure used to elect the Bavarian state parliament. It is a form of open-list MMP (where "MMP" can also be called "Additional Member System"). Like all MMP methods, it elects some fixed number L of winners in local districts (1 per district), while G other winners are chosen to ensure global proportionality.

Here is the procedure:

  1. Each ballot has two parts: local and regional. On the local part, voters can choose one of the local candidates (and, implicitly, also partially support the party of that candidate); on the regional part, voters can choose one of the non-local candidates (and, implicitly, also partially support that party; which can be the same as or different from the party of the chosen local candidate). In practice, ballots generally have one column for each party, with the local candidates in the first row, then a thick line, then all the non-local candidates below that (written in an order chosen by their party).
  2. Within each district, the local candidate who got a plurality of local ballots is the winner and is seated.
  3. Each party's proportion of the global support is calculated, weighting support on each half of each ballot equally. This is used to assign the remaining global seats to parties using some proportional method. (Issues of overhang and/or thresholds must be dealt with, but the particular Bavarian rules are not described here.)
  4. Each party's global seats are given to whichever of their candidates got the most votes without winning locally.

Modified Bavarian MMP

The method described above and used in Bavaria is generally "better" than most versions of MMP in two ways:

  • As an open-list method, it has more voter choice than typical closed-list MMP variants
  • Because both halves of the ballot are weighted equally in determining party proportions, the incentives to strategically split your ballot across two parties (as in the so-called "decoy lists" that occur in, for instance, Welsh elections) are reduced.

However, there are two modifications, both inspired by STV, that can make Bavarian MMP even more robust. Both are changes to how party proportions are calculated in step 3; neither one requires a change to ballot format.

  1. If a ballot supports the winner in the local district, then the global part of that ballot is counted for the party of that local winner. (This is inspired by exhausting ballots in STV, and reduces the chances of "overhang".)
  2. If a ballot supports two different parties on its two halves, and exactly one of those two parties is nonviable (cannot win any seats), then it is counted as if both parts of that ballot supported the viable party. (This is inspired by transferring ballots in STV, and reduces the chances of "wasted/sub-threshold" voting power.)