Example - Single Transferable Vote - STV
Suppose an ice-cream "election" is conducted to determine which flavour
to serve at a
birthday party.
• There are 5 flavours
"candidates",
3 of which will be chosen.
• The
candidates are: Vanilla, Neopolitan, Chocolate, Strawberry,
and Licorice.
• The 20 guests at the party have the preferences marked on their
ballots
in the two tables below
(the first is a numerical representation, the
second is pictorial).
In this election only the first 2
preferences of the voter is shown, because in this case their lower
preferences will not affect the result.
# of Guests -->
|
x x x x |
x x |
x x x x
x x x x |
x x x x |
x |
x |
1st Preference |
|
|
|
|
|
|
2nd Preference
|
|
|
|
|
|
|
to clarify above - 4 guests (x x x x)
prefer vanilla 1st, neopolitan 2nd; 2 guests prefer neopolitan 1st,
vanilla 2nd; 8 guests want chocolate 1st, strawberry 2nd; etceteras
Next- the quota is
calculated.
Using the Droop
quota*, with 20
voters and 3 flavors (candidates) to be chosen, the number of votes
required to be
"elected" is:
-
* see and understand Droop quota as this is the difficult
Math concept
When ballots are counted the election proceeds as follows:
Candidate: |
|
|
|
|
|
Round 1 |
x x x x |
x x |
x x x x
x x x x
x x x x |
x |
x |
Round 1: Chocolate is declared elected, since
Chocolate has more votes than the quota - 6
|
Round 2 |
x x x x |
x x |
x x x x
x x |
x x x x
x |
x x x |
Round 2: Chocolate's surplus votes transfer
proportionately
to Strawberry and Licorice according to the Chocolate voters' second
choice preferences. However, even with the transfer of this surplus no
candidate has reached the quota. Therefore Neopolitan, who has the
fewest
votes, is eliminated. |
Round 3 |
x x x x
x x |
|
x x x x
x x |
x x x x
x |
x x x |
Round 3: Neopolitan's votes transfer to their second
preference, Vanilla, causing Vanilla to reach the quota and be elected.
Vanilla
barely meets the quota, and therefore has no surplus to transfer. |
Round 4 |
x x x x
x x |
|
x x x x
x x |
x x x x
x |
x x x |
Round 4: Neither of the remaining candidates meets the
quota, so Licorice is eliminated. Strawberry is the only remaining
candidate and so wins the final seat. |
Result: The winners are Chocolate, Vanilla
and Strawberry.