ICSI Quota - The Ceiling - Demystified

Quota

The quota (sometimes called the threshold) is the number of votes a candidate must receive to be elected. The Hare quota and the Droop quota are commonly used to determine the quota.

HARE Quota:

When Thomas Hare originally conceived his version of Single Transferable Vote, he envisioned using the quota:

Quota =	Votes polled
	Available Seats

In the unlikely event that each successful candidate receives exactly the same number of votes not enough candidates can meet the quota and fill the available seats in one count. Thus the last candidate cannot not meet the quota, and it may be fairer to eliminate that candidate. 

To avoid this situation, it is common instead to use the Droop quota, which is always lower than the Hare quota.

DROOP quota: (ICSI  Method)

The most common quota formula is the Droop quota which given as: 

Quota =	(Votes polled)	+ 1
	(Available Seats + 1)

Droop produces a lower quota than Hare. If each ballot has a full list of preferences, Droop guarantees that every winner meets the quota rather than being elected as the last remaining candidate after lower candidates are eliminated. The fractional part of the resulting number, if any, is dropped (the result is rounded down to the next whole number.)

It is only necessary to allocate enough votes to ensure that no other candidate still in contention could win. This leaves nearly one quota's worth of votes unallocated, but counting these would not alter the outcome.

Droop is the only whole-number threshold for which 

(a) a majority of the voters can be guaranteed to elect a majority of the seats when there is an odd number of seats; 

(b) for a fixed number of seats.

Each winner's surplus votes transfer to other candidates according to their remaining preferences.