Number of effective parties, two-party concentration ratio, and periodisation of elections
Original version: Figure 4.1 in Irish Voters Decide (p.93)
(i) The two-party concentration ratio is the sum of the share of the vote of the two largest parties following an election. With the exception of 2011 (FG + Labour), this is calculated as the sum of Fianna Fáil and Fine Gael’s vote share.
(ii) For more information on the number of effective parties, see Taagepera and Shugart (1989), Seats and Votes: The Effects and Determinants of Electoral Systems, Yale University Press: New Haven, pp. 77-81.
1922-44: Gallagher, M (ed) (1993), Irish Elections 1922-44: Results and Analysis, PSAI Press: Dublin
1948-2011: Official election returns
2016: RTÉ election results