Swiss-system tournament
A Swiss-system tournament is a competition format in which contestants are paired using rules designed to ensure that each competitor plays opponents with a similar running score without playing the same opponent more than once. Match pairing for each round is done after the previous round has ended and depends on its results. The winner is the competitor with the highest aggregate points earned in all rounds. With an even number of participants, all competitors play in each round. It contrasts with an elimination tournament, where not all participants play in later rounds, as well as with round-robin tournaments, where each competitor plays every possible opponent.
The Swiss system is used for competitions in which it is undesirable to eliminate any competitors before the end of the tournament, but which have too many entrants to make a full round-robin feasible. In contrast, all-play-all is suitable if there are a small number of competitors. The Swiss system seeks to provide a clear winner among a large number of competitors within a relatively small number of rounds of competition, while avoiding the situation in single-elimination tournaments in which a single bad result can remove a good competitor.
The system was first employed at a chess tournament in Zürich in 1895 by Julius Müller, hence the name "Swiss system", and is now used in many games including bridge, chess, and Go, among others.
Procedure
The number of rounds is fixed in advance. During all but the first round, competitors are paired based on approximately how they have performed so far. After the last round, players are ranked by their score. If players remain tied, a tie-break score is used, such as the sum of all opponents' scores.In the first round, competitors are paired either randomly or according to some pattern that has been found to serve a given game or sport well. If it is desired for top-ranked participants to meet in the last rounds, the pattern must start them in different brackets, just the same as is done in seeding of pre-ranked players for a single elimination tournament. In subsequent rounds, competitors are sorted according to their cumulative scores and are assigned opponents with the same or similar score up to that point. The pairing rules have to be quite complicated, as they have to ensure that no two players ever face each other twice, and to avoid giving a player some advantage as a result of chance.
The detailed pairing rules are different in different variations of the Swiss system. As they are quite complicated, and it is undesirable to have a long delay between rounds to decide the pairings, the tournament organizer often uses a computer program to do the pairing.
In chess, a specific pairing rule, called "Dutch system" by FIDE, is often implied when the term "Swiss" is used. The Monrad system for pairing is commonly used in chess in Denmark and Norway, as well as in other sports worldwide. These two systems are outlined below.
Dutch system
The players are divided into groups based on their scores. Within each group with the same or similar score, players are ranked based on ratings or some other criteria. Subject to the other pairing rules, the top half is then paired with the bottom half. For instance, if there are eight players in a score group, number 1 is paired with number 5, number 2 is paired with number 6 and so on. Modifications are then made to prevent competitors from meeting each other twice, and to balance colors. For this method to work, the score groups cannot be too small, and thus for smaller overall fields score groups are not a suitable approach.This pairing system may have some issues with competitive integrity if a tournament where this system is used has qualifiers leading to it. For example, suppose a certain qualifier determines the 5th-8th seeds in an 8-team Swiss-style tournament. If the Dutch system is used, players or teams in the qualifier may be incentivized to not do their best, as doing so might make them play against the 1st seed on the first round, decreasing their chances of having a good score. Conversely, for knockout tournaments, the highest seed is usually paired with the lowest, the 2nd highest with the 2nd lowest, and so on. This incentivizes players or teams to do their best and get a higher seed so that they can play against lower-seeded players/teams.
Monrad system
The players are first ranked based on their scores, then on their starting numbers. Then #1 meets #2, #3 meets #4, etc., with modifications made to ensure that other rules are adhered to. Players are sorted by scores and original ranks, then each player is paired to the next opponent, typically excluding repeats.The Monrad system used in chess in Denmark is quite simple, with players initially ranked at random, and pairings modified only to avoid players meeting each other twice. The Norwegian system has an optional seeding system for the first-round pairings, and within a score group, the pairing algorithm endeavors to give players alternating colors.
Burstein system
Like in the Dutch system, the players are divided into groups based on their scores. Within each scoregroup, players are sorted based on rating and tiebreaks. Then, the first-ranked player is paired against the last-ranked player in the scoregroup, the second-ranked to the second-to-last-ranked and so on. In a scoregroup with six players player 1 would be paired with player 6, player 2 with player 5 and player 3 with player 4.Analysis, advantages, and disadvantages
Assuming no drawn games and byes treated as wins, determining a clear winner would require the same number of rounds as that of a knockout tournament, which is the binary logarithm of the number of players rounded up. Thus, three rounds can handle up to eight players, four rounds can handle up to sixteen players, and so on. If this minimum number of rounds is not played, two or more players will always finish the tournament with a perfect score, never having faced each other. Likewise, if extra rounds are played, two or more players could finish the tournament with the same score, assuming no player received a perfect score. Due to the fact that players should meet each other at most once and pairings are chosen dependent on the results, there is a natural upper bound on the number of rounds of a Swiss-system tournament, which is equal to half of the number of players rounded up. Should more than this number of rounds be played, the tournament might run into the situation that there is either no feasible round, or some players have to play each other a second time.Unlike in a knockout tournament, a player who enters a Swiss-system tournament knows that they can play in all the rounds, regardless of results. The only exception is that one player is left over when there is an odd number of players. The player left over receives a bye: they do not play that round but are usually awarded the same number of points as for winning a game. The player is reintroduced in the next round and will not receive another bye. Another advantage is that the final ranking in a Swiss-system tournament gives some indication of the relative strengths of all contestants, not just of the tournament winner. By contrast, in a knockout tournament, the second-best contestant could have been eliminated by the eventual tournament winner in an early round and thus appear weak.
In a Swiss-system tournament, sometimes a player has such a lead that by the last round they are assured of winning the tournament even if they lose the last game. This has some disadvantages. First, a Swiss-system tournament does not always end with the exciting climax of a knockout final. Second, while the outcome of the final game does not determine first place, the first-place player's final game can determine who will be second or third. In the 1995 All-Stars Tournament in Scrabble, tournament directors paired David Gibson, who had by then clinched first place, with the highest-ranked player who could not win a prize; this allowed the second- and third-ranked players to compete between themselves for their final placements. The "Gibson Rule" is optional at Scrabble tournaments, as players at smaller tournaments may still have an incentive to win their last game to improve their overall rating. Players may also be "Gibsonized" at an earlier stage of a tournament; a player who has clinched a spot in the next round may be paired with the highest-ranked player who cannot possibly qualify for the next round.
The Swiss system is used for the selection of the English national pool team. Sixty-four players start the tournament and after six rounds, the top player will qualify as they will be unbeaten. The remaining seven places are decided after a series of round robins and playoffs.
Compared with a round-robin tournament, a Swiss tournament can handle many players without requiring an impractical number of rounds. For example, if a tennis tournament had sixty-four players, but only eight courts available, then not all matches in a round can be played at the same time. In a Swiss tournament, each round would have to be divided up into four waves of eight matches each. This would result in a total of twenty-four waves over the minimum six rounds. A single-elimination tournament also does not require an impractical number of rounds, but the best competitor might perform poorly in a single match and be eliminated early. Continuing the example, for such a tournament, the first round would require four waves, the next two, and all remaining rounds would consist of a single wave each. Over the same six rounds, only nine waves would occur. Note that the waves format is not strictly necessary, as instead a match could commence as soon as another in the same round ends, but the principle is largely the same.
In a Swiss tournament, it is necessary to have all the results of a particular round recorded before allowing the next round to begin. This means that each round will take at least as long as its slowest match. In a single elimination tournament, any game may commence once the two preceding games that feed into it have been completed. This may result in one branch of a single-elimination bracket falling behind if it has several slow matches in a row, but it may catch up if it then has several quick matches.