Condorcet ranking

A Condorcet ranking is an ordering of candidates such that:

• The Condorcet winner, if one exists, is ranked first.
• The secondary Condorcet winner (the new Condorcet winner if the Condorcet winner is eliminated), if one exists, is ranked second.
• The tertiary Condorcet winner (the new Condorcet winner if the Condorcet winner and the secondary Condorcet winner are eliminated), if one exists, is ranked third.
• etc.

and

• The Condorcet loser, if one exists, is ranked last.
• The secondary Condorcet loser (the new Condorcet loser if the Condorcet loser is eliminated), if one exists, is ranked next-to-last.
• The tertiary Condorcet loser (the new Condorcet loser if the Condorcet loser and secondary Condorcet loser are eliminated), if one exists, is ranked third-to-last.
• etc.

Smith set ranking

A Smith ranking (Smith set ranking) is the same as a Condorcet ranking, except instead of being defined in terms of the Condorcet winner and Condorcet loser, it is defined in terms of the Smith set and Smith loser set. When there is a multi-member Smith set, the candidates in the Smith set may be ranked in any order so long as they are all ranked above non-Smith set candidates, with the same applying for the Smith loser set candidates with regards to being ranked lower than non-Smith loser set candidates.

When there is a Condorcet ranking (one doesn't always exist), the Smith ranking will be the same as the Condorcet ranking. There will always be at least one Smith ranking, and it is possible to have more than one (each of which will differ only on the order in which Smith or Smith loser candidates are ranked). The simplest way to find a Smith ranking is to find the Copeland ranking (the ranking of all candidates such that the candidates with the most (pairwise victories minus pairwise defeats) are ranked first, the candidates with the second-most (pairwise victories minus pairwise defeats) are ranked second, etc.)

The generalized Smith ranking is a Smith ranking where all candidates in each consecutively ranked Smith set are equally ranked and the same holds for each consecutively ranked Smith loser set; for example:

2 A

2 B

1 C

The generalized Smith ranking is A=B>C, meaning A and B are tied for 1st place, and C is in 2nd place. Note that any of A>B=C, B>A=C, or A=B>C would be valid Smith rankings, but only the third is a generalized Smith ranking; this is because the generalized Smith ranking assumes neutrality as to who is superior within the Smith and Smith loser sets. This may make it an appropriate tool to demonstrate how various groups of candidates would compare across all Smith-efficient methods. There will always be one and only one generalized Smith set ranking.

Another example:

1 A>B>C

1 B>C>A

1 C>A>B

1 D>E

1 E>D

1 F=G

Pairwise comparison table (candidate on left is preferred by # of voters in their cell over candidate on top) Pairwise victories are bolded, defeats are underlined, ties are struck through Groups of candidates who pairwise beat all other lower groups are italicized; every odd group's numbers are made bigger and even group's numbers smaller

	A	B	C	D	E	F	G
A	-	2	1	3	3	3	3
B	1	-	2	3	3	3	3
C	2	1	-	3	3	3	3
D	2	2	2	-	1	2	2
E	2	2	2	1	-	2	2
F	1	1	1	1	1	-	0
G	1	1	1	1	1	0	-

The generalized Smith set ranking is A=B=C>D=E>F=G, or (A, B, C) tied for 1st, (D, E) tied for 2nd, and (F, G) tied for 3rd place. This is because 3 voters prefer (A, B, C) over all others (and at most 2 voters prefer any other candidates over (A, B, C)) but no smaller subset of (A, B, C) can say the same, 2 voters prefer (D, E) above all others when ignoring (A, B, C) (and at most 1 voter prefers other unignored candidates over (D, E)) but no smaller subset of (D, E) can say the same, and 1 voter prefers (F, G) above all others when ignoring (A, B, C, D, E) (and no voters prefer any other unignored candidates over (F, G)) but no smaller subset of (F, G) can say the same. Notice that when looking at the pairwise comparison table for the generalized Smith ranking that the Smith sets can be seen by looking at which groups of candidates have pairwise victories over all other candidates when ignoring the pairwise matchups between candidates in the group.

Another way of looking at the pairwise comparison table would be to put all candidates in each consecutive Smith set on their own line, like so:

	A, B, C	D, E	F, G
A, B, C	-	Win	Win
D, E	Loss	-	Win
F, G	Loss	Loss	-

The simplest way to find the generalized Smith set ranking is to find the Copeland ranking, put all candidates ranked 1st by Copeland in the Smith set and thus equally rank them 1st in the generalized Smith set ranking, and then check if any other candidates can pairwise beat or tie any of the candidates in the Smith set; if any of them can, then all candidates ranked at the lowest rank of any of them based on the Copeland ranking or above is also in the Smith set, and the process repeats until at some point, none of them can, then the candidates at the next Copeland ranking after the lowest-Copeland-ranked Smith set candidate are all part of the secondary Smith set and thus equally ranked 2nd in the generalized Smith set ranking. Repeat until all candidates are equally ranked within their consecutive Smith set.

So for example, if 3 candidates are ranked 1st in Copeland, they are part of the Smith set, and if there are several candidates ranked lower than them by Copeland who pairwise beat or tie any of them, take the lowest ranked of them based on Copeland (suppose that candidate was ranked 5th by Copeland) and add all candidates ranked 5th or above by Copeland into the Smith set. Supposing no candidates pairwise beat or tie any of the candidates newly added into the Smith set, all candidates ranked 6th by Copeland are part of the secondary Smith set, and the process repeats to see who is part of the secondary Smith set, tertiary Smith set, etc.

Notes

Methods that produce only Condorcet rankings may include Kemeny-Young; methods that produce Smith rankings and thus also Condorcet rankings when they exist include Copeland's method, Pairwise Sorted Methods, Schulze, and Ranked Pairs;

The generalized Smith ranking can be found by finding all candidates in the Smith set, equally ranking each of them, then eliminating all of them and finding the secondary Smith set (the Smith set now that the original Smith set has been eliminated) and equally ranking each of them lower than the candidates in the Smith set, and repeating until all candidates are ranked.

Smith or Condorcet rankings can be visualized in the following form:

			wi	JS	DS	KW	BK	AM
	AM	Andy Montroll (5–0)						5 Wins ↓
	BK	Bob Kiss (4–1)					1 Loss → ↓ 4 Wins	4067 (AM) – 3477 (BK)
	KW	Kurt Wright (3–2)				2 Losses → 3 Wins ↓	4314 (BK) – 4064 (KW)	4597 (AM) – 3668 (KW)
	DS	Dan Smith (2–3)			3 Losses → 2 Wins ↓	3975 (KW) – 3793 (DS)	3946 (BK) – 3577 (DS)	4573 (AM) – 2998 (DS)
	JS	James Simpson (1–4)		4 Losses → 1 Win ↓	5573 (DS) – 721 (JS)	5274 (KW) – 1309 (JS)	5517 (BK) – 845 (JS)	6267 (AM) – 591 (JS)
	wi	Write-in (0–5)	5 Losses →	3338 (JS) – 165 (wi)	6057 (DS) – 117 (wi)	6063 (KW) – 163 (wi)	6149 (BK) – 116 (wi)	6658 (AM) – 104 (wi)

With generalized Smith rankings in particular, some rows would have to be groups of candidates, rather than solely individual candidates. Hypothetical example:

			Pl	Sq	Sp, Pa, Sa
	Sp, Pa, Sa	SpongeBob, Patrick, Sandy (2-0)			5 Wins ↓
	Sq	Squidward (1-3)		3 Losses → 1 Win ↓
	Pl	Plankton (0-4)	5 Losses →	10 (Sq) – 9 (Pl)

This makes it more ambiguous as to how to record the exact margins of some pairwise matchups where groups of candidates are involved, as even if all candidates in a group of candidates pairwise beat all candidates not in the group, they may each do so with different margins.

Technical information:

Note that a variant of generalized Smith rankings can be created to address the unanimity/Pareto criterion. For example:

34 A>B>C>D

33 B>C>D>A

33 C>D>A>B

A pairwise beats B pairwise beats C pairwise beats D pairwise beats A, so all candidates here are in the Smith set. However, C is unanimously preferred to D, so the Pareto-satisfying generalized Smith ranking would acknowledge that D must be at a lower rank than C, whereas the regular generalized Smith ranking would simply be A=B=C=D.