Electoral concentration is a measure of the accumulation of votes in the two most voted parties of a given territorial unit. It is the cumulative fraction of valid votes obtained by the two most voted parties in a given election (Consejo Nacional Electoral - Ecuador, 2014).
The formula is:
$$concentration = p1 + p2$$
where p1 is the vote/seat proportion for the most voted party,
and p2 is the vote/seat proportion for the second most voted party.
Thus, it requires elections with at least 2 parties.
The index is a number positive up to 1. A value of 1 means maximum concentration (p1 + p2 = 1, i.e. pi = 0 for i = 3,4...)
Developed by Jorge Albuja Delgado (albuja@yahoo.com).