Round-robin tournament

A round-robin tournament or all-play-all tournament is a type of tournament "in which each contestant meets all other contestants in turn".[1] In a single round-robin schedule, each participant plays every other participant once. If each participant plays all others twice, this is frequently called a double round-robin. The term is rarely used when all participants play one another more than twice, 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 III and All America Football Conference for exceptions).

A round-robin tournament with four players is sometimes called a "quad". In croquet and tennis clubs in Great Britain, a round-robin tournament is often called an American tournament. The term round-robin is derived from the French term ruban, meaning "ribbon". Over a long period of time, the term was corrupted and idiomized to robin.

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Use

In sports with a large number of competitive matches per season, double round-robins are common. Most football (soccer) leagues in the world are organized on a double round-robin 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 round-robin chess and go tournaments; the World Chess Championship was decided in 2005 and in 2007 by an eight-player double round-robin 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 round-robin basis. Examples with pure round-robin scheduling include the FIFA World Cup, UEFA European Football Championship and UEFA Cup (since 2004–05) in football, the Super 14 of rugby union in the Southern Hemisphere, the Cricket World Cup and many American Football college conferences, such as the Pacific-10. The group phase of the UEFA Champions League is contested as a double round-robin, as are most basketball leagues outside the United States, including the regular-season and Top 16 phases of the Euroleague.

Evaluation

In theory a round robin tournament is the fairest way to determine a champion among a known and fixed number of participants. Each player or team has an equal chance against all other participants. The element of luck is seen to be reduced as compared to a knockout system since a few bad performances need not cripple a competitor's chances of ultimate victory. A participant's final record is thus seen to be more accurately represented in the results since it was arrived at over a prolonged 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 multi-tiered league. In English football, although the FA Cup was founded before the Football League, the (round-robin) League champions have always been regarded as the "best" team in the land, rather than the (knockout) Cup winners.

The primary disadvantage to 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 less than the number of participants. For instance, a 32 team tournament can be completed in just 5 rounds in a knockout format. However if the same teams are put through a round robin it would require 31 rounds 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. Indeed it is possible for the title to be clinched with games left to play, thus rendering remaining games meaningless to all competitors. Conversely, the ability to recover from defeats, while rewarding overall consistency, may also be seen as a crutch for competitors who lack the temperament to handle the pressure of a knockout tournament.

Further issues arise where a round-robin 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 next-phase opponent for a lower-placed qualifier is perceived to be easier than for a higher-placed one).

Swiss system tournaments attempt to combine elements of the round-robin and elimination formats, to provide a reliable champion using fewer rounds than a round-robin, 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 result.

Scheduling algorithm

If n is the number of competitors, a pure round robin tournament requires \begin{matrix} \frac{n}{2} \end{matrix}(n - 1) games. If n is even, then in each of (n - 1) rounds, \begin{matrix} \frac{n}{2} \end{matrix} games can be run in parallel, provided there exist sufficient resources (e.g. courts for a tennis tournament). If n is odd, there will be n rounds with \begin{matrix} \frac{n - 1}{2} \end{matrix} games, and one competitor having no game in that round.

The standard algorithm for round-robins 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 competitor (number one in this example) and rotate the others clockwise …

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. 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 fulfill 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.

Footnotes

  1. ↑ Webster's Third New International Dictionary of the English Language, Unabridged (1971, G. & C. Merriam Co), p.1980.

See also

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