Rankings

We use the TrueSkill system for player rankings. TrueSkill is the ranking system used on Xbox Live; it’s a bit like the ELO rankings that have been used for chess players for many years, but more sophisticated, and it supports multi-player and team games.

Players are ranked by their order of elimination in the game, so getting knocked out first is worse than getting knocked out last. In the event of a mutual titan kill, the players involved tie.

TrueSkill features two internal variables that describe the player’s ranking. mu is the estimated skill level, and theta is the range of uncertainty about the skill level. mu goes up with wins and down with losses, factoring in the skill of the opponents. theta tends to go down over time. The visible TrueSkill ranking is mu – 3 * theta, rounded down to the next lowest integer, with a floor of 1.

We currently use the default values of mu (25) and theta (8.33333333). That gives new players a visible TrueSkill of 1.

Game results and rankings are stored on the server, in a simple SQLite database, in file ~/.slugathon/results/slugathon.db

We use Heungsub Lee’s trueskill library. It was chosen over the other TrueSkill libraries because it’s in Python, available in PyPI for easy installation, and has a GPLv2-compatible license (BSD).