SOS
Sum of Opponents' Scores (SOS)
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SOS is a tie breaker often used in go tournaments. It stands for "Sum of Opponents' Scores". Chess players dub it Buchholz or Solkoff.
SOS is the sum of the scores of all opponents that a player played against in a tournament. Its purpose is to give an indication of how strong the field of opponents you faced was. It is vulnerable to noise, especially by unlucky pairings in earlier rounds.
When to use SOS
If you absolutely have to break ties and there is no option to have a play-off between tied players, then SOS or variants thereof are among the recommended tiebreakers of organisations like the EGF, the Pandanet Tour, and AGA. SOS is also often the default tie breaker of many pairing programs.
Further information and discussion regarding the merits of SOS can be found at /Discussion and Theory behind SOS.
Calculating SOS in various tournament systems
Because a player's "score" will mean different things in different tournament formats, the instructions on exactly how to calculate SOS are on their own page.
Variants
Variants of SOS include:
- SOS-1 drop the numerically smallest opponent's score from the sum. (Known to Chess as Buchholz Cut 1)
- SOS-2 drop the numerically smallest and the next smallest opponents' scores from the sum. (Known to Chess as Buchholz Cut 2)
- Median drop the smallest and the largest opponents' scores from the sum.
These methods are believed to reduce the effects of statistical noise, and pairing luck, with SOS-2 being more effective that SOS-1. SOS-1 and SOS-2 become more effective at reducing noise in tournaments with more rounds.
These variants are rarely used, as support for them in pairing programs is limited or non-existent.
Other Considerations
Unplayed games will cause trouble with SOS. See the SOS calculation page for a more detailed explanation and recommended countermeasures.