Swiss-system tournament

A Swiss system tournament is a commonly used type of tournament in chess, bridge, Scrabble, squash, Magic: The Gathering and other games where players or teams need to be paired to face each other. This type of tournament was first used in a Zurich chess tournament in 1895, hence the name "Swiss system".

The pairing procedure

The principle of a Swiss tournament is that each player will be pitted against another player who has done as well (or as poorly) as him or herself.

The first round is either drawn at random or seeded according to rating. Players who win receive a point, those who draw receive half a point and losers receive no points. Win, lose, or draw, all players proceed to the next round where winners are pitted against winners, losers are pitted against losers, and so on. In subsequent rounds, players face opponents with the same (or almost the same) score. No player is paired up against the same opponent twice however. In chess it is also attempted to ensure that each player plays an equal number of games with white and black, alternate colors in each round being the most preferable, and a concerted effort is made not to assign the same color three times in a row.

The basic rule is that players with the same score are ranked according to rating. Then the top half is 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 balance colors and prevent players from meeting each other twice.

The detailed rules of how to do the pairing are usually quite complicated and often the tournament organizer has access to a computer to do the pairing for him. If the rules are strictly adhered to, the organizer has no discretion in pairing the round. See the link below for detailed pairing rules from FIDE.

Final scores and tie-breaking

The tournament lasts for a number of rounds announced before the tournament. After the last round, players are ranked by their score. If this is tied then a tie break score (such as the sum of all their opponents' scores) or the Buchholz chess rating can be used: see Tie-breaking in Swiss system tournaments.

Analysis, advantages, and disadvantages

Determining a clear winner (and, incidentally, a clear loser) usually requires the same number of rounds as a knockout tournament, that is the Binary logarithm of the number of players rounded up. Therefore three rounds can handle eight players, four rounds can handle sixteen players and so on, however it is not uncommon to have more players than this, and, if fewer than the ideal number of rounds are played, it can happen that two or more players finish the tournament with a perfect score, having won all their games but never faced each other.

Compared to a knockout tournament the Swiss system has the inherent advantage of not eliminating anyone. That means that a player can enter such a tournament knowing that he will be able to play in all rounds, regardless of how well he does. The worst that can happen in this respect is being the player left over when there is an odd number of players. The player left over receives a bye, meaning he does not play that particular round but receives a full point as if he had won a game. He is reintroduced in the next round and will not receive another bye.

Another advantage compared to knockout tournaments is that the final ranking gives some indication of relative strength for all contestants, not just for the winner of the tournament. As an example, the losing finalist in a knockout tournament may not be the second best contestant; that might have been any of the contestants eliminated by the eventual tournament winner in earlier rounds.

A Swiss system tournament does not always end with the exciting climax of the knockout's final however. Sometimes a player may have picked up such a great lead that by the last round he is assured of winning the tournament even if he loses the last game. One fairly common fix for this dilemma is to hold single elimination rounds among the top scorers. In Scrabble tournaments a player with such a strong lead will often be paired against the highest-placed player who cannot possibly finish in the prize-winning zone; this process is known as Gibsonization after it was first applied to the US Champion David Gibson in the 1995 All-Stars tournament. He is the all-time top money winner in the history of Scrabble, and has made a habit of clinching victory in major events without waiting for the final round. Because of this, players are said to be Gibsonized when after winning, they are paired with lower-ranked players to avoid affecting the ranking of runners-up.

Compared with a round-robin tournament, a Swiss can handle many players without requiring an impractical number of rounds. An elimination tournament is better suited to a situation in which only a limited number of games may be played at once, e.g. tennis. In a Swiss system, all players can be playing a round at the same time.

Variations of the Swiss system

Accelerated pairings

The method of accelerated pairings also known as accelerated Swiss is used in some large tournaments with more than the optimal number of players for the number of rounds. This method pairs top players more quickly than the standard method in the opening rounds and has the effect of reducing the number of players with perfect scores more rapidly .

For the first two rounds, players who started in the top half have one point added to their score for pairing purposes only. Then the first two rounds are paired normally, taking this added score into account. In effect, in the first round the top quarter plays the second quarter and the third quarter plays the fourth quarter. Most of the players in the first and third quarters should win the first round. Assuming this is approximately the case, in effect for the second round the top eighth plays the second eighth, the second quarter plays the third quarter and the seventh eighth plays the bottom eighth. That is, in the second round, winners in the top half play each other, losers in the bottom half play each other, and losers in the top half play winners in the bottom half (for the most part). After two rounds, about ⅛ of the players will have a perfect score, instead of ¼. After the second round, the standard pairing method is used (without the added point for the players who started in the top half).

As a comparison between the standard Swiss system and the accelerated pairings, consider a tournament with eight players, ranked #1 through #8. Assume that the higher-ranked player always wins.

Standard Swiss system

Round 1:
#1 plays #5, #1 wins
#2 plays #6, #2 wins
#3 plays #7, #3 wins
#4 plays #8, #4 wins

Round 2:
#1 plays #3, #1 wins
#2 plays #4, #2 wins
#5 plays #7, #5 wins
#6 plays #8, #6 wins

After two rounds, the standings are:
1 2-0
2 2-0
3 1-1
4 1-1
5 1-1
6 1-1
7 0-2
8 0-2

Accelerated pairings

Round 1:
#1 plays #3, #1 wins
#2 plays #4, #2 wins
#5 plays #7, #5 wins
#6 plays #8, #6 wins

Round 2:
#1 plays #2, #1 wins
#3 plays #5, #3 wins
#4 plays #6, #4 wins
#7 plays #8, #7 wins

After two rounds, the standings are:
1 2-0
2 1-1
3 1-1
4 1-1
5 1-1
6 1-1
7 1-1
8 0-2

McMahon system

A variant known as the McMahon system tournament is the established way in which European Go tournaments are run. This differs mainly in that players start at different levels; so the Swiss system is the special case where all players start at the same level. It is named for Lee E. McMahon (1931–1989) of Bell Labs.

Konrad system

In a few tournaments which run over a long period of time, such as a tournament with one round every week for three months, a flexible system called a Konrad tournament can be used. A player's final score is based on his best results (e.g. best ten results out of the twelve rounds). Players are not required to play in every round, they may enter or drop out of the tournament at any time. Indeed they may decide to play only one game if they wish to, although if a player wants to get a prize they need to play more rounds to accumulate points. The tournament therefore includes players who want to go for a prize and play several rounds as well as players who only want to play an off game. This system is used by a few chess clubs in Norway.

Monrad system

A common tournament system in Norway and Denmark is the Monrad system. This is very similar to the Swiss System, but deemphasizes ratings, and bases the pairings on the starting number each contestant has received at random before the tournament.

The Danish version is a fairly simple method, players are initially ranked at random, and pairings are modified mainly 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 endeavours to give players alternating colors.

Magic card game

The DCI, the tournament sanctioning body for the card game Magic: The Gathering uses a Swiss system for most tournaments. Unlike with other Swiss implementations, players receive three points for a win and only one for a draw. After a number of rounds sufficient to provide one undefeated player, as detailed above, in some tournaments the top eight players advance to single-elimination, with several statistics used as tie-breakers.

A side effect of this system is that in the second-to-last round, the four undefeated players can agree to draw the remaining rounds, and doing so usually grants them a position in the top eight. The extra point awarded for a win serves to discourage this however.

Scrabble

In some Scrabble tournaments, a system known variously as "modified Swiss", "Portland Swiss", "Fontes Swiss" or "speed pairing" is used, whereby first players are placed in groups of four, and play three rounds of round-robin play, and subsequently are paired as in Swiss pairing, but using the standings as of the second to last round, rather than the last round. This has the advantage of allowing the tournament directors to already know who plays whom by the time given players are finished with a round, rather than making the players wait until all players have finished playing a given round before being able to start the time-consuming pairing process.

Commonly used in Australia, and now in many other countries, is a system known as "Australian Draw". Whereby each round is paired using a normal #1 plays #2, #3 plays #4, etc. except that repeat pairings within a selected range of previous games is forbidden. Often, for shorter tournaments the selected range will be since the very first round of the tournament, thus never having a repeat pairing for the entire tournament. For longer tournaments it is also common to have the first N rounds use the Australian Draw system, and followed by one or more "King Of the Hill" rounds. "King Of the Hill" is a strict #1 plays #2, #3 plays #4, etc. with no regard to previous pairings, thus unlimited repeat pairings are allowed.

Another Scrabble system based on Swiss pairing is known as "Chew pairing", and has been used at recent North American National and Canadian National Scrabble championships, and since 2005 at the World Scrabble Championship. Simulations are performed to determine which players are still in contention for each prize and those players are paired so as to balance the right of a low-ranked player to avoid elimination by challenging a high-ranked player with the right of high-ranked players to compete directly with each other for prizes.

Bridge

The Swiss system is also used in some bridge tournament events. They involve teams of four, five, or six players (usually four). In each round, one team plays against another one for several hands, with scoring by International Match Points. These "IMP's" are converted to victory points, with either twenty or thirty victory points split between the two teams. In the first round, teams are paired essentially randomly. In subsequent rounds, the teams are ranked in order of the number of victory points they have accumulated, and the top team plays the second team, the third team plays the fourth team, etc, subject to the proviso that teams don't play each other twice.

Debate tabs

British Parliamentary Style debate competitions have four rather than two teams in each debate. The preliminary round for many such competitions, including the World Universities Debating Championship, ranks teams by a modified form of Swiss tournament, usually called a tab. "Tab" also denotes to the software used for scheduling of rounds and tabulation of results. Teams are ranked from first to fourth in each debate and awarded from three down to zero points. Teams with similar points totals are grouped off for each successive round. Just as chess Swiss tournaments are arranged to ensure players have a balance of playing with black pieces and white pieces, so too debate tournaments attempt to provide teams with a balance of places in the speaking order (i.e. Opening Government, Opening Opposition, Closing Government, and Closing Opposition). With four competitors rather than two, significantly greater compromise is required to balance the ideal requirements of, on the one hand, a team not meeting the same opponent twice and, on the other hand, a team having a balanced mix of places in the running order.

Other tournament systems

• Round-robin tournament
• Single-elimination tournament
• Double-elimination tournament

See also

• Tie-breaking in Swiss system tournaments

External links

• Detailed rules from FIDE on the Swiss pairing system
• Comparison of Swiss and Round Robin formats