Calculates the measure of two-partiness D2
This function calculates two-partiness (two-partyness) D2, as introduced by Gaines and Taagepera. The argument is a vector with the seat shares of the parties. See
GTT for the other measure introduced by Gaines and Taaepera.
Gaines, Brian J., and Rein Taagepera. 2013. How to Operationalize Two-Partyness. Journal of Elections, Public Opinion & Parties. doi:10.1080/17457289.2013.770398.