Dual Member Proportional (DMP) is not used anywhere in the world. It came third in the Prince Edward Island referendum of 2016 and is on the ballot for the 2018 referendum in British Columbia.
How to vote
Under DMP each constituency would elect two MPs. Parties would typically run two candidates on the same ticket, one as a primary candidate and the other as the secondary. Voters have a single vote and can cast it for a party ticket of two candidates, a party ticket of one candidate, or an independent.
How votes are counted
The primary candidate on the ticket with the largest number of votes becomes the first of the two MPs.
At this stage, the result will not match how people voted in their region. Some parties will have far too many seats and some far too few. DMP uses the second seats to make up for this.
If the second-placed candidate is an independent they get the second position.
The remaining primary candidates (and secondary candidates if their primary won the first seat) for each party go on a regional list. The list is in the order of the percent of the vote each candidate got, with any secondary candidates getting half of their ticket vote.
The counters then work out how many seats each party should have. For instance, a party that got half the vote should get half the seats. If 20 seats were available in a region and they won 7 of the first seats, they deserve 3 of the second seats.
The top 3 candidates on their list win the second seats in the constituencies they stood in.
Sometimes multiple candidates might deserve to win the same second seat. If there are conflicting claims to the seat the candidate with the most votes gets it. This process repeats until all the parties have their fair share of seats.
Effects and features
The second seats try to correct the disproportionate results. But, a party could win all the first seats with less than half the vote in each constituency, but then end up with 50% of the seats in parliament.
Only having to cast one vote may be simple, but it can have some complicated results. A voter may like their local MP but not want their party to form the government or vice versa. With a single vote, there is no way to differentiate. You may also prefer the party’s secondary candidate over their primary, but have no way of showing this.
Unlike mixed systems, DMP has a single tier of members. Though in practice the system may still encourage tension between the first seat and second seat winners. Proponents argue that it’s fairer for independent candidates than in AMS or Party List PR systems.
DMP’s system means that second candidates should enjoy some popular support in their constituency. However, it is possible that in some cases second candidates might win on relatively small percentages of the vote.
As a safeguard DMP includes a threshold of 5% that candidates must pass to win a seat. But, candidates’ position on their party list relies on the relative success of their colleagues. So, you may get occasions where a candidate will lose their seat even when their personal vote has gone up or vice versa.