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'''CloneproofThe Schwartz Sequential Dropping'''Schulze (method'''CSSD''') is a [[voting system]] developed by Markus Schulze that selects a single winner using votes that express preferences. The CSSDSchulze method can also be used to create a sorted list of winners. The CSSDSchulze method is also known as "cloneproof Schwartz Sequentialsequential Droppingdropping" (CSSD), "BeatpathSchwartz Methodsequential dropping" (SSD), "Beatpathbeatpath Winnermethod", "Pathbeatpath Votingwinner", "Pathpath Winnervoting", and "Schulzepath Methodwinner".
 
If there is a candidate who is preferred over the other candidates,
when compared in turn with each of the others, CSSDthe Schulze method guarantees that that candidate will win.
Because of this property, CSSDthe Schulze method is (by definition) a '''[[Condorcet method]]'''.
Note that this is different from some other preference voting systems such as [[Borda count|Borda]] and
[[Instant-runoff voting]], which do not make this guarantee.
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== The Schwartz Set ==
 
The definition of a [[Schwartz set]], as used in CSSDthe Schulze method, is as follows:
 
# An unbeaten set is a set of candidates of whom none is beaten by anyone outside that set.
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The voters cast their ballots by ranking the candidates according to their preferences, just like for any other Condorcet election.
 
CSSDThe Schulze method uses [[Condorcet method|Condorcet]] pairwise matchups between the candidates and a winner is chosen in each of the matchups.
 
From there, CSSDthe Schulze method operates as follows to select a winner (or create a ranked list):
 
# Calculate the Schwartz set based only on undropped defeats.
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In this situation the Schwartz set is A, B, and C as they all beat someone.
 
CSSDThe Schulze method then says to drop the weakest defeat, so we drop C > A and are left with
 
* A > B 72% (as C has been removed)
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== History ==
 
The Schulze method has been proposed by Markus Schulze in 1997. See e.g. [http://lists.electorama.com/htdig.cgi/election-methods-electorama.com/1998-August/001958.html here], [http://lists.electorama.com/htdig.cgi/election-methods-electorama.com/1998-August/002044.html here], and [http://lists.electorama.com/htdig.cgi/election-methods-electorama.com/1998-November/002771.html here]!
See http://lists.electorama.com/pipermail/election-methods-electorama.com/2003-December/011477.html
 
== Satisfied Criteria ==
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== External Resources ==
 
* [http://groups.yahoo.com/group/election-methods-list/ A mailing list containing technical discussions about election methods]
* [http://electionmethods.org/ electionmethods.org] where CSSD is also called "Schwartz Sequential Dropping" and "Beatpath Winner"
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