First database update: 2006-10-27 17:10:03 UTC
Last database update: 6 min
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Common information about ELO rating
The ELO rating system is a method for calculating the relative skill levels of players in two-player games such as chess and Go. It is also used as a rating system for competitive multiplayer play in a number of computer games. It was originally invented as an improved chess rating system in the 1960s and established at the FIDE-congress 1970 in Siegen (Germany). ELO is often written in capital letters, but is not an acronym. It is the family name of the system's creator, Árpád Élo (1903-1992), a Hungarian-born American physics professor. 'ELO' is written in uppercase to distinguish it from Professor Élo. He spelled his own name 'Elo' after he left Hungary, a common anglicization. (source: Wikipedia)

Relevance for BZFlag
This ranking method is irrelevant for the game itself. But the ELO system is used by most of the leagues for calculating player or team strength indexes. Such a strength index depends on the own and the opponent's strength index. As initial rating, players get 1200 points by default.
It's important to keep in mind the weighting factor depends on the match length. This modification is different between the leagues!
'Ducati-Style'-CTF-League :: 30min => 50 * 1.5 = 75 || 20min => 50 * 1.0 = 50 || 15min => 50 * 0.75 = 37.5
GU-League :: 30min => 50 * 1.0 = 50 || 20min => 50 * 2/3 = 33.3

ELO rating calculator & formula
Player A: 
Player B: 
Weighting factor*: 
Rounding type**: 

Player A Player B
Chance to win 50% 50%
Player A wins 1225 1175
Draw 1200 1200
Player B wins 1175 1225

E_A = 1 / ( 1 + pow( 10, ( (R_B - R_A) / 400 ) ) )
R_A_N = R_A + round( K * (S_A - E_A) )
E_A:  expected score for player A
R_A:  real/current ELO-score of player A
R_B:  real/current ELO-score of player B
*K:  weighting factor - normally 15, for top-players 10, for beginners 25
The 'Ducati-Style'-CTF-League and the GU-League use a value of 50 to get more significant changes for the scores after a match!
**round():  rounding type - Usually, the result score (resp. the score change) won't be rounded off, but sometimes it's done by necessity or intention.
The 'Ducati-Style'-CTF-League and the GU-League use the type 'down' (rounding down).
S_A:  match result of player A (1 for a victory, 0.5 for a draw, 0 for a defeat)
R_A_N:  new ELO-rating of player A (new ELO score of player B is calculated vice versa since ELO is symmetric)