Welsh Assembly MMP: not quite proportional

The results from the Welsh Assembly elections are in, with some interesting results:

Labour - 29.6%, 26 seats (43% of seats)
Plaid Cymru - 21.0%, 15 seats (25% of seats)
Conservative - 21.5%, 12 seats (20% of seats)
Lib Dems - 11.7%, 6 seats (10% of seats)
local independent - 1 seat
others - 16.2%, 0 seats

The facts that most jump out are:
(1) Labour won a disproportionately large share of seats, despite the MMP system, and
(2) Plaid placed ahead of the Conservatives despite a slightly smaller regional vote share.

Both of these issues stem from characteristics of the MMP system that Wales (and Scotland) use for elections. The Welsh system has 60 seats, with 40 elected in single-member constituencies and 20 elected from the five regions (four per region) as compensatory list seats. The two features that lead to the outcomes noted above are:
(1) The system does not compensate for overhangs. Every region has 11-13 seats, of which 4 are compensatory and the rest are from single-member constituencies. Taking South Wales West as an example, Labour won all 7 single-member districts (63 percent of the seats already) but took only 36 percent of the PR vote, meaning it deserved only 5 of the 11 seats. In the version of MMP used by Germany or New Zealand, Labour could keep the 7 seats, but an additional 6 seats would be awarded to other parties, increasing the size of the legislature by two. Not so in Wales (or Scotland)--the extra seats are scrapped, meaning fewer seats for the others.
(2) Compensatory seats are only 1/3 of the Assembly. Enlarging compensatory seats to 1/2 of the Assembly, even if overhangs are still not included, would mostly eliminate the problem.


The overhangs described in (1) give Labour five extra seats in the Welsh Assembly. The big loser was the Conservatives. In the regions where Labour took extra seats in constituencies, the Conservatives were generally the party that lost the compensatory seat which was no longer allocated. This happened in North Wales, South Wales West and South Wales East. (The Lib Dems also lost two seats in this way).

If overhangs were incorporated into the electoral system, the results would have been:
Labour - 29.6%, 26 seats (40% of seats)
Conservative - 21.5%, 15 seats (23% of seats)
Plaid Cymru - 21.0%, 15 seats (23% of seats)
Lib Dems - 11.7%, 8 seats (12% of seats)
local independent - 1 seat
others - 16.2%, 0 seats
The Assembly would be enlarged from 60 to 65 seats.

Labour would still enjoy a disproportionate share of seats, but not by quite a large margin.