Share on Facebook Tweet on Twitter Share on LinkedIn Share by email
TrueSkill™ Ranking System
TrueSkill™ Ranking System

The TrueSkill™ ranking system is a skill based ranking system for Xbox Live developed at Microsoft Research.

The TrueSkill ranking system is a skill based ranking system for Xbox Live developed at Microsoft Research. The purpose of a ranking system is to both identify and track the skills of gamers in a game (mode) in order to be able to match them into competitive matches. The TrueSkill ranking system only uses the final standings of all teams in a game in order to update the skill estimates (ranks) of all gamers playing in this game. Ranking systems have been proposed for many sports but possibly the most prominent ranking system in use today is ELO.

Ranking Players

So, what is so special about the TrueSkill ranking system? In short, the biggest difference to other ranking systems is that in the TrueSkill ranking system skill is characterised by two numbers:

  • The average skill of the gamer (μ in the picture).
  • The degree of uncertainty in the gamer's skill (σ in the picture).

The ranking system maintains a belief in every gamer's skill using these two numbers. If the uncertainty is still high, the ranking system does not yet know exactly the skill of the gamer. In contrast, if the uncertainty is small, the ranking system has a strong belief that the skill of the gamer is close to the average skill.

On the right hand side, a belief curve of the TrueSkill ranking system is drawn. For example, the green area is the belief of the TrueSkill ranking system that the gamer has a skill between level 15 and 20.

Maintaining an uncertainty allows the system to make big changes to the skill estimates early on but small changes after a series of consistent games has been played. As a result, the TrueSkill ranking system can identify the skills of individual gamers from a very small number of games. The following table gives an idea of the average number of games per gamer that the system ideally needs to identify the skill level:

Game Mode

Number of Games per Gamer 

16 Players Free-For-All 

8 Players Free-For-All 

4 Players Free-For-All 

2 Players Free-For-All 

12 

4 Teams/2 Players Per Team 

10 

4 Teams/4 Players Per Team 

20 

2 Teams/4 Players Per Team 

46 

2 Teams/8 Players Per Team 

91 

The actual number of games per gamer can be up to three times higher depending on several factors such as the variation of the performance per game, the availability of well-matched opponents, the chance of a draw, etc. If you want to learn more about how these numbers are calculated and how the TrueSkill ranking system identifies players' skills, please read the Detailed Description of the TrueSkill™Ranking Algorithm or find out in the Frequently Asked Questions.

Matching Players

If you play a ranked game on Xbox Live, the TrueSkill ranking system will compare your individual skill (the numbers μ and σ) with the skills of all the game hosts for that game mode on Xbox Live and automatically match you with players with skill similar to your own. But how can this be done when every player's skill is represented by two numbers? The trick is to use the (hypothetical) chance of drawing with someone else: If you are likely to draw with another player then that player is a good match for you! Sounds simple? It is!

Publications