[EM] Single-winner election data from the OpenSTV database

[Greg]

There are 65 real single winner elections in OpenSTV Database
(stv.sourceforge.net/stvdb).While the database doesn't list the
individual ballots, it does list the Smith Set for each of these
races. In 63 of these 65 races, there exists a dominant Condorcet
winner (the Smith set has a size of 1).

For 61 of the 65 races, the database also includes the results of
applying other single-winner methods to the ballots, including IRV,
Coombs, Borda, Supplementary Vote, Plurality, and Bucklin. 60 of these
61 have a Condorcet winner. Here is the Smith efficiency (frequency of
electing a candidate from the Smith Set) for each of the applied
methods, from best to worst:

IRV, Coombs: 100% Smith Efficiency (61 of 61 races)

Borda, Supplementary Vote: 96.7% Smith Efficiency (59 of 61 races)

Plurality: 95.1% Smith Efficiency (58 of 61 races)

Bucklin Voting: 91.8% Smith Efficiency (56 out of 61 races)

- Greg