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Roundrobin tournament
A roundrobin tournament (or allplayall tournament) is a competition "in which each contestant meets all other contestants in turn".^{[1]}^{[2]} It contrasts with an elimination tournament.
Contents
Terminology
The term roundrobin is derived from the term ruban, meaning "ribbon". Over a long period of time, the term was corrupted and idiomized to robin.^{[3]}^{[4]}
In a single roundrobin schedule, each participant plays every other participant once. If each participant plays all others twice, this is frequently called a double roundrobin. The term is rarely used when all participants play one another more than twice,^{[1]} and is never used when one participant plays others an unequal number of times (as is the case in almost all of the major United States professional sports leagues – see AFL (1940–41) and AllAmerica Football Conference for exceptions).
In the United Kingdom, a roundrobin tournament is often called an American tournament in sports such as tennis or billiards which usually have knockout tournaments.^{[5]}^{[6]}^{[7]} In Italian it is called girone all'italiana (literally "Italianstyle circuit"). In Serbian it is called the Berger system (Бергеров систем, Bergerov sistem), after chess player Johann Berger. A roundrobin tournament with four players is sometimes called "quad" or "foursome".^{[8]}
Use
In sports with a large number of competitive matches for season, double roundrobins are common. Most association football leagues in the world are organized on a double roundrobin basis, in which every team plays all others in its league once at home and once away. This system is also used during qualification for major tournaments such as the FIFA World Cup and the respective continental tournaments (e.g. UEFA European Championship, CONCACAF Gold Cup, etc.). There are also roundrobin chess, draughts, go, curling and Scrabble tournaments. The World Chess Championship decided in 2005 and in 2007 on an eightplayer double roundrobin tournament where each player faces every other player once as white and once as black.
Group tournaments rankings usually go by number of matches won and drawn, with any of a variety of tiebreaker criteria.
Frequently, pool stages within a wider tournament are conducted on a roundrobin basis. Examples with pure roundrobin scheduling include the FIFA World Cup, UEFA European Football Championship and UEFA Cup (2004–2005) in football, Super Rugby (rugby union) in the Southern Hemisphere during its past incarnations as Super 12 and Super 14 (but not in the current 15team format), the Cricket World Cup, Indian Premier League Twenty20 Cricket and many American Football college conferences, such as the Big 12 (which currently has 10 members). The group phases of the UEFA Champions League and Copa Libertadores de América are contested as a double roundrobin, as are most basketball leagues outside the United States, including the regularseason and Top 16 phases of the Euroleague; the United Football League has used a double roundrobin for both its 2009 and 2010 seasons.
Evaluation
In theory, a roundrobin tournament is the fairest way to determine the champion among a known and fixed number of participants. Each participant, player or team, has equal chances against all other opposites. Luck element is seen to be reduced as compared to a knockout system since bad performances need not cripple a competitor's chance of ultimate victory. Final records of participants is, thus, seen to be more accurately as they represent the results since it was arrived at over a longer period against equal competition. This can also be used to determine which teams are the poorest performers and thus subject to relegation if the format is used in a multitiered league. For example, in team sport the (roundrobin) major league champions is generally regarded as the "best" team in the land, rather than the (elimination) cup winners.
The main disadvantage of a round robin tournament is the time needed to complete it. Unlike a knockout tournament where half of the participants are eliminated after each round, a round robin requires one round less than the number of participants if the number of participants is even, and as many rounds as participants if the number of participants is odd. For instance, a tournament of 16 teams can be completed in just 4 rounds (i.e. 15 matches) in a knockout format. However if the same teams are put through a single roundrobin it would require 15 rounds (i.e. 120 matches) to finish. Other issues stem from the difference between the theoretical fairness of the round robin format and practice in a real event. Since the victor is gradually arrived at through multiple rounds of play, teams who perform poorly can be eliminated from title contention rather early on, yet they are forced to play out their remaining games. Thus games occur late in competition between competitors with no remaining chance of success. Moreover, some later matches will pair one competitor who has something left to play for against another who does not. This asymmetry means that playing the same opponents is not necessarily equitable: the same opponents in a different order may play harder or easier matches. Teams may also suffer injuries to their star players during competition and thus a matchup may have a completely different complexion than it would have if the order of play was different. There is also no showcase final match.
Further issues arise where a roundrobin is used as a qualifying round within a larger tournament. A competitor already qualified for the next stage before its last game may either not try hard (in order to conserve resources for the next phase) or even deliberately lose (if the scheduled nextphase opponent for a lowerplaced qualifier is perceived to be easier than for a higherplaced one). Four pairs in the 2012 Olympics Women's doubles badminton having qualified for the next round, were disqualified for attempting to lose in the round robin stage to avoid compatriots and better ranked opponents.^{[9]} The round robin stage at the Olympics were a new introduction and potential problems were readily known prior to the tournament.
Swiss system tournaments attempt to combine elements of the roundrobin and elimination formats, to provide a reliable champion using fewer rounds than a roundrobin, while allowing draws and losses. Also if the tournament is not held at a true neutral location and is instead at a team's home field or away the system of Double Round Robin is an effective equalizer. In this format each team plays each other twice, once away and once at home, in an effort to account for meetings of teams where homefield could sway the results.
Scheduling algorithm
If <math>n</math> is the number of competitors, a pure round robin tournament requires <math>\begin{matrix} \frac{n}{2} \end{matrix}(n  1)</math> games. If <math>n</math> is even, then in each of <math>(n  1)</math> rounds, <math>\begin{matrix} \frac{n}{2} \end{matrix}</math> games can be run concurrently, provided there exist sufficient resources (e.g. courts for a tennis tournament). If <math>n</math> is odd, there will be <math>n</math> rounds, each with <math>\begin{matrix} \frac{n  1}{2} \end{matrix}</math> games, and one competitor having no game in that round.
The standard algorithm for roundrobins is to assign each competitor a number, and pair them off in the first round …
Round 1. (1 plays 14, 2 plays 13, ... ) 1 2 3 4 5 6 7 14 13 12 11 10 9 8
then fix one of the competitors in the first or last column of the table (number one in this example) and rotate the others clockwise one position
Round 2. (1 plays 13, 14 plays 12, ... ) 1 14 2 3 4 5 6 13 12 11 10 9 8 7
Round 3. (1 plays 12, 13 plays 11, ... ) 1 13 14 2 3 4 5 12 11 10 9 8 7 6
until you end up almost back at the initial position
Round 13. (1 plays 2, 3 plays 14, ... ) 1 3 4 5 6 7 8 2 14 13 12 11 10 9
If there are an odd number of competitors, a dummy competitor can be added, whose scheduled opponent in a given round does not play and has a bye. The schedule can therefore be computed as though the dummy were an ordinary player, either fixed or rotating. Instead of rotating one position, any number relatively prime to <math>(n1)</math> will generate a complete schedule. The upper and lower rows can indicate home/away in sports, white/black in chess, etc.; to ensure fairness, this must alternate between rounds since competitor 1 is always on the first row. If, say, competitors 3 and 8 were unable to fulfil their fixture in the third round, it would need to be rescheduled outside the other rounds, since both competitors would already be facing other opponents in those rounds. More complex scheduling constraints may require more complex algorithms.^{[10]} This schedule is applied in chess and draughts tournaments of rapid games, where players physically move round a table. In France this is called the CarouselBerger system (Système RutchBerger).^{[11]}
Alternatively Berger tables,^{[12]} named after the Austrian chess master Johann Berger, are widely used in the planning of tournaments. Berger published the pairing tables in his two Schachjahrbucher,^{[13]}^{[14]} with due reference to its inventor Richard Schurig.^{[15]}^{[16]}
Round 1. 1–14 2–13 3–12 4–11 5–10 6–9 7–8 Round 2. 14–8 9–7 10–6 11–5 12–4 13–3 1–2 Round 3. 2–14 3–1 4–13 5–12 6–11 7–10 8–9 … Round 13. 7–14 8–6 9–5 10–4 11–3 12–2 13–1
This constitutes a schedule where player 14 has a fixed position, and all other players are rotated clockwise <math>\begin{matrix} \frac{n}{2}\end{matrix}</math> positions. This schedule alternates colours and is easily generated manually. To construct the next round, the last player, number 8 in the first round, moves to the head of the table, followed by player 9 against player 7, player 10 against 6, until player 1 against player 2. Arithmetically, this equates to adding <math>\begin{matrix} \frac{n}{2}\end{matrix}</math> to the previous row, with the exception of player <math>n</math>. When the result of the addition is greater than <math>(n1)</math>, then subtract <math>(n1)</math>.
This schedule can also be represented as a (n1, n1) table, expressing a round in which players meets each other. For example player 7 plays against player 11 in round 4. If a player meets itself, then this shows a bye or a game against player n. All games in a round constitutes a diagonal in the table.


The above schedule can also be represented by a graph, as shown below:
Both the graph and the schedule were reported by Édouard Lucas in ^{[17]} as a recreational mathematics puzzle. Lucas, who describes the method as simple and ingenious attributes the solution to Felix Walecki, teacher at Lycée Condorcet. Lucas also shows an alternative solution by means of a sliding puzzle.
Original construction of pairing tables by Richard Schurig (1886)
For 7 or 8 players, Schurig^{[16]} builds a table with <math>n/2</math> vertical rows and <math>n1</math> horizontal rows, as follows:
1.  Round  1  2  3  4 
2.  ,,  5  6  7  1 
3.  ,,  2  3  4  5 
4.  ,,  6  7  1  2 
5.  ,,  3  4  5  6 
6.  ,,  7  1  2  3 
7.  ,,  4  5  6  7 
Then a second table is constructed (with counting from the end) as shown below:
1.  Round  . 1  . 7  . 6  . 5 
2.  ,,  . 5  . 4  . 3  . 2 
3.  ,,  . 2  . 1  . 7  . 6 
4.  ,,  . 6  . 5  . 4  . 3 
5.  ,,  . 3  . 2  . 1  . 7 
6.  ,,  . 7  . 6  . 5  . 4 
7.  ,,  . 4  . 3  . 2  . 1 
By merging above tables we arrive at:
1.  Round  1, 1  2, 7  3, 6  4, 5 
2.  ,,  5, 5  6, 4  7, 3  1, 2 
3.  ,,  2, 2  3, 1  4, 7  5, 6 
4.  ,,  6, 6  7, 5  1, 4  2, 3 
5.  ,,  3, 3  4, 2  5, 1  6, 7 
6.  ,,  7, 7  1, 6  2, 5  3, 4 
7.  ,,  4, 4  5, 3  6, 2  7, 1 
Then the first column is updated: if <math>n</math> is even, player number <math>n</math> is alternatingly substituted for the first and second positions, whereas if <math>n</math> is odd a bye is used instead.
The pairing tables were published as an annex concerning the arrangements for the holding of master tournaments. Schurig did not provide a proof nor a motivation for his algorithm. For more historical details, see Ahrens.^{[18]}
See also
 Group tournament ranking system, including details of tiebreaking systems
 Combinatorial design, a balanced tournament design of order n (a BTD(n))
 Tournament (graph theory), mathematical model of a roundrobin tournament
 Other tournament systems:
 Bridge:
 Chess:
 List of roundrobin chess tournaments
 Scheveningen system, where each member of one team plays each member of the other
External links
 Round Robin Discussion Board link to a discussion community and schedules (balanced,cyclic,firstfit,whist).
 Round Robin System Tables (Scoring  Programme)
References
 ^ ^{a} ^{b} Webster's Third New International Dictionary of the English Language, Unabridged (1971, G. & C. Merriam Co), p.1980.
 ^ Orcutt, William Dana (1895). Official Lawn Tennis Bulletin 2. New York: The Editors. pp. 1, 3.
 ^ Strehlov, Richard A; Wright, Sue Ellen, eds. (1993). "Standardizing Terminology for Better Communication: Practice, Applied Theory, and Results" 1166. ASTM. pp. 336–337. ISBN 0803114931.
 ^ Brewer's Dictionary of Phrase & Fable. New York: Harper & Brother Publishers. p. 786.
 ^ "A Glossary of Terms Used in Connection with Billiards". Billiard Monthly (English Amateur Billiards Association). February 1912.
American Tournament: A tournament in which each player must meet in turn every other player.
 ^ Allied. "American tournament". Chambers 21st Century Dictionary. Allied Publishers. p. 38. ISBN 9780550106254. Retrieved 1 August 2012.
 ^ Mead, Shepherd (1977). How to succeed in tennis without really trying: the easy tennismanship way to do all the things no tennis pro can teach you. McKay. p. 130. ISBN 9780679507499. Retrieved 1 August 2012.
 ^ "An Introduction to USCFRated Tournaments" (PDF). The United States Chess Federation. 23 Feb 2006.
 ^ "Eight Olympic badminton players disqualified for 'throwing games'". The Guardian. 1 August 2012. Retrieved 1 August 2012.
 ^ Dinitz, Jeff (13 November 2004). "Designing Schedules for Leagues and Tournaments" (PDF). Home Page for Jeff Dinitz. Mount Saint Mary College: GRAPH THEORY DAY 48.
 ^ Le livre de l'arbitre : édition 2008 (PDF) (in French). Fédération Française des Échecs. p. 56. ISBN 9782915853018.
 ^ Table de Berger Invalid language code., examples of round robin schedules up to 30 participants.
 ^ Berger, Johann (1893). SchachJahrbuch für 1892/93 (in German). Leipzig. OCLC 651254787.
 ^ Berger, Johann (1899). SchachJahrbuch für 1899/1900 : fortsetzung des schachjahrbuches für 1892/93 (in German). Leipzig. pp. 21–27. OCLC 651254792.
 ^ Richard Schurig Invalid language code.
 ^ ^{a} ^{b} Schurig, Richard (1886). "Die Paarung der Theilnehmer eines Turniers". Deutsche Schachzeitung (in German) 41: 134–137. OCLC 556959107.
 ^ Lucas, Edouard (1883). "Les jeux de demoiselles". Récréations Mathématiques (in French). Paris: GauthierVillars. pp. 161–197.
 ^ Ahrens, Wilhelm (1901). "Anordnungs Probleme, Aufgabe 2". Mathematische Unterhaltungen und Spiele (in German). Leipzig: B. G. Teubner. ark:/13960/t2w37mv93.