Single-elimination tournament

Example of a single-elimination tournament bracket

A single-elimination tournament—also called an Olympic system tournament, a Bump-Off, a knockout (or, knock-out), single penetration, or sudden death tournament—is a type of elimination tournament where the loser of each bracket is immediately eliminated from winning the championship or first prize in the event.

This does not always mean that the defeated competitor will not participate further in the tournament; in some such tournaments, consolation or "classification" contests are subsequently held among those already defeated to determine the awarding of lesser places, for example, a Third place playoff. Where more than two competitors can play in each match, such as in a shootout poker tournament, players are removed when they can no longer play until one player remains from the group. This player moves on to the next round.

Some competitions are held with a pure single-elimination tournament system. Others have many phases, with the last being a single-elimination final stage called playoffs.

Nomenclature

In English, the round in which only eight competitors remain is generally called the quarter-final round; this is followed by the semi-final round, in which only four are left, the two winners of which then meet in the final or championship round. The round before the quarter-finals is sometimes called the "round of sixteen", "Last Sixteen", or (in South Asia) "pre-quarterfinals"; in some other languages "eighth-finals" is used. Earlier rounds are typically numbered counting forwards from the first round, or by the number of remaining competitors. If some competitors get a bye, the round at which they enter may be named the "first" round, with the earlier matches called a "preliminary" round, or "play-in games".

Examples of the names given to rounds in various generic conventions and specific competitions:

Notes:

Example

The final three rounds of the 2014 Australian Open – Women's Singles knock-out tournament:

	Quarterfinals							Semifinals							Final
	14	Ana Ivanovic	7	5	2
	30	Eugenie Bouchard	5	7	6
								30	Eugenie Bouchard	2	4
								4	Li Na	6	6
	4	Li Na	6	6
	28	Flavia Pennetta	2	2
															4	Li Na	77	6
															20	Dominika Cibulková	63	0
	11	Simona Halep	3	0
	20	Dominika Cibulková	6	6
								20	Dominika Cibulková	6	6
								5	Agnieszka Radwańska	1	2
	5	Agnieszka Radwańska	6	5	6
	2	Victoria Azarenka	1	7	0

Classification

When matches are held to determine places or prizes lower than first and second (the loser of the final-round match gaining the latter position), these typically include a match between the losers of the semifinal matches called Third place playoffs, the winner therein placing third and the loser fourth. Many Olympic single-elimination tournaments feature the bronze medal match if they do not award bronze medals to both losing semifinalists. The FIFA World Cup has long featured the third place match, though the UEFA Euro has not held one since the 1980 edition.

Sometimes, contests are also held among the losers of the quarterfinal matches to determine fifth to eighth places – this is most commonly encountered in the Olympic Games, with the exception of boxing, where both fighters are deemed to be third place. In one scenario, two "consolation semifinal" matches may be conducted, with the winners of these then facing off to determine fifth and sixth places and the losers playing for seventh and eighth; those are used often in qualifying tournaments where only the top five teams advance to the next round; or some method of ranking the four quarterfinal losers might be employed, in which case only one round of additional matches would be held among them, the two highest-ranked therein then playing for fifth and sixth places and the two lowest for seventh and eighth.

The number of distinct ways of arranging a single-elimination tournament (as an abstract structure, prior to seeding the players into the tournament) is given by the Wedderburn–Etherington numbers.^[5] Thus, for instance, there are three different arrangements for five players:

• The players may be divided into brackets of two and three players, the winners of which meet in the final game
• The bottom four players may play a two-round tournament, the winner of which plays the top player
• The bottom two players may meet, after which each subsequent game pairs the winner of the previous game with the next player

However, the number of arrangements grows quickly for larger numbers of players and not all of them are commonly used.

Seeding

Opponents may be allocated randomly (such as in the FA Cup); however, since the "luck of the draw" may result in the highest-rated competitors being scheduled to face each other early in the competition, seeding is often used to prevent this. Brackets are set up so that the top two seeds could not possibly meet until the final round (should both advance that far), none of the top four can meet prior to the semifinals, and so on. If no seeding is used, the tournament is called a random knockout tournament.

One version of seeding is where brackets are set up so that the quarterfinal pairings (barring any upsets) would be the 1 seed vs. the 8 seed, 2 vs. 7, 3 vs. 6 and 4 vs. 5; however, this is not the procedure that is followed in most tennis tournaments, where the 1 and 2 seeds are placed in separate brackets, but then the 3 and 4 seeds are assigned to their brackets randomly, and so too are seeds 5 through 8, and so on. This may result in some brackets consisting of stronger players than other brackets, and since only the top 32 players are seeded at all in Tennis Grand Slam tournaments, it is conceivable that the 33rd-best player in a 128-player field could end up playing the top seed in the first round. A good example of this occurring was when World No. 33 Florian Mayer was drawn against (and eventually defeated by) then-World No. 1 Novak Djokovic in the first round of the 2013 Wimbledon Championships,^[6] in what was also a rematch of a quarterfinal from the previous year.^[7] While this may seem unfair to a casual observer, it should be pointed out that rankings of tennis players are generated by computers, and players tend to change ranking positions very gradually, so that a more equitable method of determining the pairings might result in many of the same head-to-head matchups being repeated over and over again in successive tournaments.

Sometimes the remaining competitors in a single-elimination tournament will be "re-seeded" so that the highest surviving seed is made to play the lowest surviving seed in the next round, the second-highest plays the second-lowest, etc. This may be done after each round, or only at selected intervals. In American team sports, for example, the MLS, NFL and WNBA employ this tactic, but the NBA does not (and neither does the NCAA college basketball tournament). MLB does not have enough teams (10) in its playoff tournament where re-seeding would make a large difference in the matchups; (MLS and NFL are at the minimum, which is six from each conference [or league in MLB] for a total of 12). The NBA's format calls for the winner of the first-round series between the first and eighth seeds (within each of the two conferences the league has) to face the winner of the first-round series between the fourth and fifth seeds in the next round, even if one or more of the top three seeds had been upset in their first-round series; critics have claimed that this gives a team fighting for the fifth and sixth seeding positions near the end of the regular season an incentive to tank (deliberately lose) games, so as to finish sixth and thus avoid a possible matchup with the top seed until one round later.

In some situations, a seeding restriction may be implemented; from 1975 until 1989 in the NFL, and from 1994 until 2011 in MLB there was a rule where at the conference or league semifinal, should the top seed and last seed (wild card) be from the same division, they could not play each other; in that case, the top seed plays the worst division champion; the second-best division champion plays the wild card team. This is due to the scheduling employed for the regular season, in which a team faces any given divisional opponent more often than any given non-divisional opponent – the tournament favors matchups that took place fewer times in the regular season (or did not take place, in some cases).

In international fencing competitions, it is common to have a group stage. Participants are divided in groups of 6–7 fencers who play a round-robin tournament, and a ranking is calculated from the consolidated group results. Single elimination is seeded from this ranking.

Evaluation

The single-elimination format enables a relatively large number of competitors to participate. There are no "dead" matches (perhaps excluding "classification" matches), and no matches where one competitor has more to play for than the other.

The format is less suited to games where draws are frequent. In chess, each fixture in a single-elimination tournament must be played over multiple matches, because draws are common, and because white has an advantage over black. In association football, games ending in a draw may be settled in extra time and eventually by a penalty shootout or by replaying the fixture.

Another perceived disadvantage is that most competitors are eliminated after relatively few games. Variations such as the double-elimination tournament allow competitors a single loss while remaining eligible for overall victory.

In a random knockout tournament (single-elimination without any seeding), awarding the second place to the loser of the final is unjustified: any of the competitors knocked out by the tournament winner might have been the second strongest one, but they never got the chance to play against the losing finalist. In general, it is only fair to use a single-elimination tournament to determine first place. To fairly determine lower places requires some form of round-robin in which each player/team gets the opportunity to face every other player/team.

Also, if the competitors' performance is variable, that is, it depends on a small, varying factor in addition to the actual strength of the competitors, then not only will it become less likely that the strongest competitor actually wins the tournament, in addition the seeding done by the tournament organizers will play a major part in deciding the winner.^[8] As a random factor is always present in a real-world competition, this might easily cause accusations of unfairness.

Other tournament systems

Variations of the single-elimination tournament include:

• Double-elimination tournament
• McIntyre System, a group of tournament formats that combine features of single- and double-elimination tournaments. Varieties of this system include:
  • Page playoff system (four teams)
  • Top five play-offs
  • Top six play-offs
  • McIntyre Final Eight System
  • Super League play-offs, which formerly used a McIntyre Final Eight variant
  • AFL final eight system, another variant of the McIntyre Final Eight, currently used by the Australian Football League

Other common tournament types include:

• Round-robin tournament
• Swiss-system tournament

References