I’ve alluded to the phenomenon of the “seat bonus” or “bounce” in a previous review of the results from the recent general election, where careful vote management by parties can bring in extra candidates under the PR-STV system. This is particularly true of larger more established parties. Here is a much fuller description of how this effected the outcome of the 2016 ballot, from Seán Donnelly over on RTÉ:
“Under our Proportional Representation Single Transferable Vote system a party should get an equal share of seats to votes. In other words if a party gets 20% of the first-preference vote they should get 20% of the seats.
This would give a % Seats to % Votes Ratio of 100%.
I have used this seats to votes ratio as a Performance Indicator (PI) for the various parties.
Fine Gael had a poor election in 2016 but were saved by their vote management. They were just one point ahead of Fianna Fáil on first preferences which would have given them just a one seat advantage but a better bonus seats performance saw them winning twice as many bonus seats to edge the gap to six seats.
Thus Fine Gael went close to their excellent 2011 performance with a ratio of 124%, their second best ever.
Fianna Fáil also did well and were above their historical average and above their 2014 locals ratio. But good vote management gave Fine Gael two seats or more in 15 constituencies with Fianna Fáil winning two in just nine and that was the main difference between the two largest parties.
Sinn Féin got one bonus seat for a positive ratio of 105%, which was down on their local election ratio of 110%.
The Others as expected fell well short of what their first preference vote warranted as they took 13 seats less for a ratio of just 67%, down from 89% in the locals.
None of the smaller parties came up to scratch and were all below the 100% bench mark from AAA-PBP best at 96% to The Green Party at just 47% (excluding Renua).”
This is how that translates into percentages and Teachtaí Dála: