Later-no-help criterion
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The later-no-help criterion is a voting system criterion formulated by Douglas Woodall. The criterion is satisfied if, in any election, a voter giving an additional ranking or positive rating to a less-preferred candidate can not cause a more-preferred candidate to win. Voting systems that fail the later-no-help criterion are vulnerable to the tactical voting strategy called mischief voting, which can deny victory to a sincere Condorcet winner.
Complying methods[edit]
Two-round system, Single transferable vote (including traditional forms of Instant Runoff Voting and Contingent vote), Approval voting, Borda count, Range voting, Bucklin voting, and Majority Judgment satisfy the later-no-help criterion.
When a voter is allowed to choose only one preferred candidate, as in plurality voting, later-no-help can either be considered satisfied (as the voter's later preferences can not help their chosen candidate) or not applicable.
Noncomplying methods[edit]
All Minimax Condorcet methods (including the pairwise opposition variant), Ranked Pairs, Schulze method, Kemeny-Young method, Copeland's method, Nanson's method, and Descending Solid Coalitions, a variant of Woodall's Descending Acquiescing Coalitions, do not satisfy later-no-help. The Condorcet criterion is incompatible with later-no-help.
Checking Compliance[edit]
Checking for failures of the Later-no-help criterion requires ascertaining the probability of a voter's preferred candidate being elected before and after adding a later preference to the ballot, to determine any increase in probability. Later-no-help presumes that later preferences are added to the ballot sequentially, so that candidates already listed are preferred to a candidate added later.
Examples[edit]
Anti-plurality[edit]
Anti-plurality elects the candidate the fewest voters rank last when submitting a complete ranking of the candidates.
Later-No-Help can be considered not applicable to Anti-Plurality if the method is assumed to not accept truncated preference listings from the voter. On the other hand, Later-No-Help can be applied to Anti-Plurality if the method is assumed to apportion the last place vote among unlisted candidates equally, as shown in the example below.
Truncated Ballot Profile[edit]
Assume four voters (marked bold) submit a truncated preference listing A > B = C by apportioning the possible orderings for B and C equally. Each vote is counted A > B > C, and A > C > B:
# of voters | Preferences |
---|---|
2 | A ( > B > C) |
2 | A ( > C > B) |
4 | B > A > C |
3 | C > B > A |
Result: A is listed last on 3 ballots; B is listed last on 2 ballots; C is listed last on 6 ballots. B is listed last on the least ballots. B wins. A loses.
Adding Later Preferences[edit]
Now assume that the four voters supporting A (marked bold) add later preference C, as follows:
# of voters | Preferences |
---|---|
4 | A > C > B |
4 | B > A > C |
3 | C > B > A |
Result: A is listed last on 3 ballots; B is listed last on 4 ballots; C is listed last on 4 ballots. A is listed last on the least ballots. A wins.
Conclusion[edit]
The four voters supporting A increase the probability of A winning by adding later preference C to their ballot, changing A from a loser to the winner. Thus, Anti-plurality fails the Later-no-help criterion when truncated ballots are considered to apportion the last place vote amongst unlisted candidates equally.
Coombs' method[edit]
Coombs' method repeatedly eliminates the candidate listed last on most ballots, until a winner is reached. If at any time a candidate wins an absolute majority of first place votes among candidates not eliminated, that candidate is elected.
Later-No-Help can be considered not applicable to Coombs if the method is assumed to not accept truncated preference listings from the voter. On the other hand, Later-No-Help can be applied to Coombs if the method is assumed to apportion the last place vote among unlisted candidates equally, as shown in the example below.
Truncated Ballot Profile[edit]
Assume four voters (marked bold) submit a truncated preference listing A > B = C by apportioning the possible orderings for B and C equally. Each vote is counted A > B > C, and A > C > B:
# of voters | Preferences |
---|---|
2 | A ( > B > C) |
2 | A ( > C > B) |
4 | B > A > C |
4 | C > B > A |
2 | C > A > B |
Result: A is listed last on 4 ballots; B is listed last on 4 ballots; C is listed last on 6 ballots. C is listed last on the most ballots. C is eliminated, and B defeats A pairwise 8 to 6. B wins. A loses.
Adding Later Preferences[edit]
Now assume that the four voters supporting A (marked bold) add later preference C, as follows:
# of voters | Preferences |
---|---|
4 | A > C > B |
4 | B > A > C |
4 | C > B > A |
2 | C > A > B |
Result: A is listed last on 4 ballots; B is listed last on 6 ballots; C is listed last on 4 ballots. B is listed last on the most ballots. B is eliminated, and A defeats C pairwise 8 to 6. A wins.
Conclusion[edit]
The four voters supporting A increase the probability of A winning by adding later preference C to their ballot, changing A from a loser to the winner. Thus, Coombs' method fails the Later-no-help criterion when truncated ballots are considered to apportion the last place vote amongst unlisted candidates equally.
Copeland[edit]
This example shows that Copeland's method violates the Later-no-help criterion. Assume four candidates A, B, C and D with 7 voters:
Truncated preferences[edit]
Assume that the two voters supporting A (marked bold) do not express later preferences on the ballots:
# of voters | Preferences |
---|---|
2 | A |
3 | B > A |
1 | C > D > A |
1 | D > C |
The results would be tabulated as follows:
X | |||||
A | B | C | D | ||
Y | A | [X] 3 [Y] 3 |
[X] 2 [Y] 5 |
[X] 2 [Y] 5 | |
B | [X] 3 [Y] 3 |
[X] 2 [Y] 3 |
[X] 2 [Y] 3 | ||
C | [X] 5 [Y] 2 |
[X] 3 [Y] 2 |
[X] 1 [Y] 1 | ||
D | [X] 5 [Y] 2 |
[X] 3 [Y] 2 |
[X] 1 [Y] 1 |
||
Pairwise election results (won-tied-lost): | 2-1-0 | 2-1-0 | 0-1-2 | 0-1-2 |
Result: Both A and B have two pairwise wins and one pairwise tie, so A and B are tied for the Copeland winner. Depending on the tie resolution method used, A can lose.
Express later preferences[edit]
Now assume the two voters supporting A (marked bold) express later preferences on their ballot.
# of voters | Preferences |
---|---|
2 | A > C > D |
3 | B > A |
1 | C > D > A |
1 | D > C |
The results would be tabulated as follows:
X | |||||
A | B | C | D | ||
Y | A | [X] 3 [Y] 3 |
[X] 2 [Y] 5 |
[X] 2 [Y] 5 | |
B | [X] 3 [Y] 3 |
[X] 4 [Y] 3 |
[X] 4 [Y] 3 | ||
C | [X] 5 [Y] 2 |
[X] 3 [Y] 4 |
[X] 1 [Y] 3 | ||
D | [X] 5 [Y] 2 |
[X] 3 [Y] 4 |
[X] 3 [Y] 1 |
||
Pairwise election results (won-tied-lost): | 2-1-0 | 0-1-2 | 2-0-1 | 1-0-2 |
Result: B now has two pairwise defeats. A still has two pairwise wins, one tie, and no defeats. Thus, A is elected Copeland winner.
Conclusion[edit]
By expressing later preferences, the two voters supporting A promote their first preference A from a tie to becoming the outright winner (increasing the probability that A wins). Thus, Copeland's method fails the Later-no-help criterion.
Dodgson's method[edit]
Dodgson's' method elects a Condorcet winner if there is one, and otherwise elects the candidate who can become the Condorcet winner after the fewest ordinal preference swaps on voters' ballots.
Later-No-Help can be considered not applicable to Dodgson if the method is assumed to not accept truncated preference listings from the voter. On the other hand, Later-No-Help can be applied to Dodgson if the method is assumed to apportion possible rankings among unlisted candidates equally, as shown in the example below.
Truncated Ballot Profile[edit]
Assume ten voters (marked bold) submit a truncated preference listing A > B = C by apportioning the possible orderings for B and C equally. Each vote is counted A > B > C, and A > C > B:
# of voters | Preferences |
---|---|
5 | A ( > B > C) |
5 | A ( > C > B) |
10 | B > A > C |
2 | C > B > A |
1 | C > A > B |
Against A | Against B | Against C | |
---|---|---|---|
For A | 11 | 20 | |
For B | 12 | 15 | |
For C | 3 | 8 |
Result: B is the Condorcet winner and the Dodgson winner. A loses.
Adding Later Preferences[edit]
Now assume that the ten voters supporting A (marked bold) add later preference C, as follows:
# of voters | Preferences |
---|---|
10 | A > C > B |
10 | B > A > C |
2 | C > B > A |
1 | C > A > B |
Against A | Against B | Against C | |
---|---|---|---|
For A | 11 | 20 | |
For B | 12 | 10 | |
For C | 3 | 13 |
Result: There is no Condorcet winner. A is the Dodgson winner, because A becomes the Condorcet Winner with only two ordinal preference swaps (changing B > A to A > B). A wins.
Conclusion[edit]
The ten voters supporting A increase the probability of A winning by adding later preference C to their ballot, changing A from a loser to the winner. Thus, Dodgson's method fails the Later-no-help criterion when truncated ballots are considered to apportion the possible rankings amongst unlisted candidates equally.
Ranked pairs[edit]
For example, in an election conducted using the Condorcet compliant method Ranked pairs the following votes are cast:
28: A | 42: B>A | 30: C |
A is preferred to C by 70 votes to 30 votes. (Locked)
B is preferred to A by 42 votes to 28 votes. (Locked)
B is preferred to C by 42 votes to 30 votes. (Locked)
B is the Condorcet winner and therefore the Ranked pairs winner.
Suppose the 28 A voters specify second choice C (they are burying B).
The votes are now:
28: A>C | 42: B>A | 30: C |
A is preferred to C by 70 votes to 30 votes. (Locked)
C is preferred to B by 58 votes to 42 votes. (Locked)
B is preferred to A by 42 votes to 28 votes. (Cycle)
There is no Condorcet winner and A is the Ranked pairs winner.
By giving a second preference to candidate C the 28 A voters have caused their first choice to win. Note that, should the C voters decide to bury A in response, B will beat A by 72, restoring B to victory.
Similar examples can be constructed for any Condorcet-compliant method, as the Condorcet and later-no-help criteria are incompatible.
Commentary[edit]
Woodall writes about Later-no-help, "... under STV [single transferable vote] the later preferences on a ballot are not even considered until the fates of all candidates of earlier preference have been decided. Thus a voter can be certain that adding extra preferences to his or her preference listing can neither help nor harm any candidate already listed. Supporters of STV usually regard this as a very important property, although not everyone agrees; the property has been described (by Michael Dummett, in a letter to Robert Newland) as 'quite unreasonable', and (by an anonymous referee) as 'unpalatable.'"^{[1]}
See also[edit]
References[edit]
- ^ Woodall, Douglas, Properties of Preferential Election Rules, Voting matters - Issue 3, December 1994
Further reading[edit]
- Tony Anderson Solgard and Paul Landskroener, Bench and Bar of Minnesota, Vol 59, No 9, October 2002. [1]
- Brown v. Smallwood, 1915