Pairwise counting: Difference between revisions
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==== Dealing with unmarked/last-place candidates ====
Note that when a candidate is unmarked it is generally treated as if the voter has no preference between the unmarked candidates (a candidate who is marked on the ballot is considered '''explicitly''' supported, and a candidate who is unmarked is '''implicitly''' unsupported). When the voter has no preference between certain candidates, which can also be seen by checking if the voter ranks/scores/marks multiple candidates in the same way (i.e. they say two candidates are both their 1st choice, or are both scored a 4 out of 5), then it is treated as if the voter wouldn't give a vote to any of those candidates in their matchups against each other.
== Dealing with write-in candidates ==
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==Negative vote-counting approach==
[[File:Negative vote-counting approach to pairwise counting.png|thumb|1114x1114px|Negative vote-counting approach for pairwise counting (Note: Regular approach may be better in some use cases; see cited discussions in text to the left).]]
[[File:Pairwise counting negative counting with ranked ballot GIF.gif|thumb|454x454px|GIF for negative counting. Click on the image and then the thumbnail of the image to see the animation.]]There are two steps to the negative counting approach: the information captured by the vote-counters, and the math done to find the final result.
# Vote-counting: The precinct vote-counters count the following values for a given candidate:
Note that this is faster when voters rank only a few of all candidates, and slower otherwise. For example, a voter who votes A>B when there are 10 candidates can be assumed to vote for A and B in every matchup, except they don't prefer B>A. Usually, this would require manually marking those positive preferences, resulting in 9 marks to show A being preferred to all other candidates, and 8 marks to show B preferred to all candidates except A. But negative counting only requires 3 marks: 1 each for A and B to indicate they are preferred in every matchup, and 1 to indicate that this isn't the case for B>A. ▼
#* The number of voters who ranked/rated/marked a candidate on their ballot.
#* In each head-to-head matchup, the number of voters who ''explicitly'' ranked that candidate '''below''' the other candidate ("explicitly" meaning they also marked both candidates on their ballot).
# Math: The final number of votes for the first candidate against the second candidate in each head-to-head matchup is then found by subtracting the second value for the first candidate from the first value.
'''Note''': When using this approach, there is an important caveat when dealing with voters who explicitly rank two candidates equally; see the [[#Dealing with equal-ranking]] section below.
It is not necessary to mark that a voter ranked a candidate if they ranked that candidate as their last choice, because this means they wouldn't vote for that candidate in any matchups. This advice is less relevant when write-ins are allowed, however, because even if a voter ranks a candidate last among the candidates named on their ballot, they are still implicitly ranking that candidate above all write-in candidates they didn't rank on their ballot. ▼
=== Example ===
If 10 voters vote A>B and 5 voters vote B, then A is explicitly marked on 10 ballots and B on 15, with B being explicitly ranked below A on 10 ballots, and A being explicitly ranked below B on 0 ballots. The number of votes in favor of each candidate is then:
* A>B is (10-0)=10 votes.
* B>A is (15-10)=5 votes.
=== Dealing with equal-ranking ===
The negative counting approach, depending on implementation, can
1 B>A
5 A=B </blockquote>then either it can be marked that A wins against B by:
* 2 votes to 1
* 7 votes to 6
This is because the voters who equally ranked A and B can be considered to, in the A vs B matchup, either be voting for:
* Neither of them (similar to [[FPTP|choose-one FPTP voting]]; this is the traditional pairwise counting approach).
* Both of them (similar to [[Approval voting]]).
This is related to how, in [[Approval voting]], if A has 30 approvals and B 20, and no other information is supplied, then it is impossible to know whether the 20 voters who approved B also approved A or not. This can change who wins if using the [[Winning votes|winning-votes]] version of a [[:Category:Defeat-dropping Condorcet methods|Category:Defeat-dropping Condorcet method]], because not only does it matter who wins the matchup, but also exactly how many voters genuinely preferred the winner to the loser.
==== Example of the two approaches to equal-ranking ====
If a voter had ranked 9 of 10 candidates as their 1st choices, and the 10th candidate was unranked (i.e. implicitly ranked last), then at least 9 marks must be made in both approaches, to indicate that 1 voter has ranked each of the 9 candidates who are the voter's 1st choice. In addition:
===== Explicitly equal-ranked candidates both get a vote =====
No extra work needs to be done.
===== Equal-ranked candidates don't get votes =====
For each matchup, the following number of markings can be made for two candidates A and B:
* 2 markings can be made (1 negative vote for A>B and 1 for B>A).
* 1 negative marking can be made for the A vs B matchup in general, which is later interpreted as a negative vote for both candidates.
In this example, there are 0.5*(9*8)=0.5*72='''36''' matchups to count between equally-ranked candidates. Accordingly, either 36*2='''72''' or '''36''' markings can be made.
=== Dealing with last-place candidates ===
It is not necessary to mark that a voter ranked a candidate if they ranked that candidate as their last choice, because this means they wouldn't vote for that candidate in any matchups.
==== Write-in candidates ====
▲
=== Comparison to the regular approach ===
Comparison between the regular approach and negative counting:
* '''The regular approach''': The precinct vote-counters manually count all of the voter's preferences in each head-to-head matchup; in other words, a candidate is assumed to be preferred only in the matchups where the vote-counters mark them as being so.
** This can be slow, and also can make it difficult to accommodate write-in candidates (see the above section), since the vote-counters won't know ahead of time who those candidates are, and thus won't be able to indicate preferences in those matchups.
* '''Negative counting approach''': The vote-counters mark a candidate as being ranked on a ballot, assume the voter who marked them prefers that candidate in every matchup, and then show which matchups this is not true for.
▲Note that this negative counting is faster when voters rank only a few of all candidates, and potentially slower otherwise. For example, a voter who votes A>B when there are 10 candidates can be assumed to vote for A and B in every matchup, except they don't prefer B>A. Usually, this would require manually marking those positive preferences, resulting in 9 marks to show A being preferred to all other candidates, and 8 marks to show B preferred to all candidates except A. But negative counting only requires 3 marks: 1 each for A and B to indicate they are preferred in every matchup, and 1 to indicate that this isn't the case for B>A.
=== Connection to cardinal methods ===
This approach can be considered an [[Approval voting]]-based or [[cardinal]] approach, because when explicit equal-rankings are counted as a vote for both candidates in the matchup, then each voter that votes Approval-style (i.e. explicitly ranks some candidates 1st and all others implicitly last) will have their ballot counted like an [[Approval ballot]] (i.e. all approved candidates receive one mark, and all disapproved candidates receive no marks).
==== Using with strength of preference ====
Negative vote-counting can be used to count weak pairwise preferences (i.e. if a voter only wants to give 0.4 votes in a matchup, rather than 1 vote; see [[Rated pairwise preference ballot#Implementations]]) by counting only a "partial ballot" marking a candidate, and partial (i.e. weighted or fractional) negative votes in certain matchups. In other words, it is treated as if only a partial voter
==References==
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