One of the expectations of the reformers who advocated replacing Colombia’s former de-facto SNTV (single nontransferable vote) system with list-PR using D’Hondt divisors (and of the foreign political scientists who advised them) was that party-system fragmentation would be reduced.
In the first use of the new system in 2006, there was change, but not much. The effective number of seat-winning parties (Ns) in the single 100-seat district for the Senate fell from 9.33 in 2002 to 7.18 in 2006. That’s not a trivial drop, but 7.18 would still be “too high” by the normative standards likely to be used by almost any electoral reformer.
Now, in the just-concluded second run of the new system, the effective number of seat-winning parties in the Senate was 5.43.
It seems the reform is working.
1. Already in 2006, we saw expected effects of the reform, even if they did not show up in this common comparative measure of electoral fragmentation. There were many fewer one-seat parties in 2006 (2) than in 2002 (42!). (I guess “many less” is an understatement!) The reason the effective number did not change much was that the size of the largest party fell from 28 in 2002 (Liberal) to 20 in 2006 (La U).
2. The effective number of seat-winners in 2002 was actually greater than the effective number of vote-winning parties (Nv). The latter fell from 8.93 in 2002 to 8.07 in 2006. It is very unusual for Ns>Nv, and that it was so in 2002 was a product of how easy it is to win a seat under high-magnitude SNTV with very small vote shares.
3. In a surprising way, the inability of Uribe to run for a third term might have helped produce greater concentration on a few large parties in 2010. In 2006, as in 2002, Uribe ran as an independent, which complicated the coordination of legislative candidacies around a smaller number of parties–especially given that legislative elections occur before presidential (by eleven weeks). In 2010, the various partisan options for succession to Uribe are clearer, even if the two main ones (La U and Conservative) are competing in the presidential election this time rather than endorsing the same one, as in 2006. (The main opposition party, Liberal, was the same size in 2010 as in 2006.)
4. Here are the preliminary seat totals, presented as 2010 seats, 2006 seats, party name (corrected):
28, 20, La U
23, 18, Conservative
17, 17, Liberal
9, 0, PIN
8, 15, Cambio Radical
8, 11, Polo Democratico
5, 0, Verde
2, 0, MIRA
0, 7, Convergencia Popular Civica
0, 5, Alas Equipo Colombia
0, 3, Colombia Democratica
0, 2, Colombia Viva
0, 2, others