# [EM] PR-STV Strategic Vulnerability

raphfrk at netscape.net raphfrk at netscape.net
Fri Dec 15 08:48:28 PST 2006

``` One strategic vulnerability of PR-STV is vote management. This is where a party tries to ensure that most of their first choice votes are split equally between all their candidates in the hope that the quota will drop.

For example, assume a 4 seater

37A1>A2
27 B
18 C1>C2
18 C2>C1

The quota is (100)/5 = 20 (actually slightly larger than 20)

Round 1:
A1: 37
A2: 0
B: 27
C1: 18
C2: 18

Round 2:

A1: 20*
A2: 17 (all transfers from A1)
B: 27
C1: 18
C2: 18

B elected and 7 votes exhausted

A1: 20*
A2: 17
B: 20*
C1: 18
C2: 18

A2 is lowest, so is eliminated

Only 4 candidates left, so winners are:

A1, B, C1, C2

18.5: A1>A2
18.5: A2>A1

then C1 or C2 would have been eliminated.

This means that coordination of voting can give a boost to some factions. The problem with this is the same as plurality, if voters' power is dependent on having to vote the party 'ticket', then it gives less choice to the voters.

Some possible solutions:

Recalculate the quota each time transfers happen.

This would mean that the surplus of elected candidates would increase and thus they would be entitled to transfer more votes.

In the above example, the total votes 'in play' at the end of round 2 is
20+17+20+18+18 = 93
The quota would thus drop to 18.6, rounded to 19.

This would allow A1 (and B) to transfer 1 more vote (actually 1.4 more votes). This would push A2 ahead of C1 and C2 and thus one of them would be eliminated. One slight issue is that this would result in B having even more wasted votes, which would reduce the quota again. This should converge, but may add a number of additional counts.

Another option would be to recalculate the quota and re-run the entire election. There are more significant convergence issues though and it means that the votes need to be counted multiple times.

Asset/PR-STV hybrid

Any exhausted votes would be transferable by the candidate who is first choice on the ballot that has become exhausted.

This would allow B to transfer his 7 vote surplus to either A2 or one of the C candidates. This is probably even fairer than recalculating the quota, as B is most likely to have his voter's interests at heart.

Asset on its own is pretty good, but judging from the number of votes that tend to transfer across party lines under PR-STV, voters are likely to prefer to have control over transfers.

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Another issue is where voters vote for candidates that they know aren't going to win as their first choice. This means that they don't waste any of their vote getting their first choice elected and can vote a full vote for their 2nd choice.

I am not sure how big an issue that is though as presumably, the people who have the same top choice as you are likely to have reasonably close 2nd choices as well.

Meek solves this, but a simplier solution would be to require that the entire election is re-run whenever a candidate is eliminated.

This means that voting for a no-hoper is a waste of time as ultimately, he will be eliminated and your vote will count for your true first choice anyway.

Raphfrk
--------------------
Interesting site
"what if anyone could modify the laws"

www.wikocracy.com
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