Droop rate
The droop quota is the most frequently used quota in elections based on the transferable individual voting procedure (STV). Sometimes it is also used in proportional representation elections with maximum numbers . In STV elections, the quota is the minimum number of votes a candidate must receive in order to be elected. All votes that a candidate receives over and above the quota are transferred to other candidates, provided that the voters have specified further candidates in their order of preference. The droop rate was developed in 1868 by the English lawyer and mathematician Henry Richmond Droop (1831-1884) as a replacement for the earlier Hare rate .
Today, the droop rate is used in almost all STV elections; this also includes the STV variants in the Republic of Ireland , Northern Ireland , Malta and Australia . The droop rate is similar to the simpler Hagenbach-Bischoff rate , which is sometimes also referred to as the “droop rate”. The formula for the droop rate is:
- valid votes = total number of valid votes cast in an election
- Seats = total number of seats to be filled in the election
Strictly speaking, the additional brackets are not necessary from a mathematical point of view, but are often used anyway to make the formula clearer for non-mathematicians. If the odds are not calculated in the correct order, the result will be incorrect.
From a mathematical point of view, the formula can best be stated as:
The square brackets indicate that it must be rounded off. In other words, the droop rate is the smallest whole number greater than votes / (seats + 1) .
The droop rate is the smallest number of votes that ensures that every candidate who receives this number of votes is actually elected. If the quota were lower, it could happen that more candidates are declared elected than there are seats to be allocated at all.
If only one seat is available, STV is the same as instant run-off voting . The droop quota then corresponds to the absolute majority of the votes.