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TrueSkill Ranking System for Matching in Games

Most multiplayer games today have some type of organization where players can come together to meet and play. This is true of all games from sports (NFL), to video games (MLG), to board games (FIDE). One of the most important roles that these organizations play is matching players with each other. The ideal matching usually depends on the relative skill of the players. Each group must choose a matching algorithm that will give them the most interesting games while taking these skills into account.

The most popular ranking system for two player games is ELO. ELO was invented as a chess ranking system but is used today for sports such as Football, Basketball, and Major League Baseball. It gives a numerical rating to each player, a higher number indicating a higher skill. After each game, the players’ skills are changed based on the results of the game and the difference in the rating of the players. Matching is done using a standard matching algorithm to pair people with similar skill levels together.

Most video games today have more than two players per game. There can be an arbitrary number of players separated into an arbitrary number of teams. The ELO ranking system is not meant to match this many players together. Microsoft’s Xbox Live is known for its excellent matchmaking, this is because they developed their own ranking algorithm known as TrueSkill. The TrueSkill ranking system maintains its “belief” of a players skill based on a Gaussian distribution, parameterized by a mean (average skill) and standard deviation (uncertainty of skill).

A TrueSkill matching is found by calculating the probability that two players (or groups of players) have the same skill, and choosing players based on those probabilities. After the game is over, each player’s ranking is updated based on their relative performance in the game. The average skill is changed based on where the player placed in the game standings. The uncertainty is decreased for players whose average skill changed as expected, and is increased uniformly in order to keep it from going to zero over many games.

The TrueSkill ranking and matching system provides Xbox Live with a competitive advantage because players are very likely to be matched with other players with similar skill levels. This leads to interesting match ups which keeps players interested in the game.

http://research.microsoft.com/en-us/projects/trueskill/

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