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Elo rating for Spell It

Elo rating for Spell It is a system for estimating relative skills level of players. A higher number indicates a higher skill. It is based on the Elo rating system commonly used in chess, as described on the wikipedia http://en.wikipedia.org/wiki/Elo_rating_system.

When a player finished a game, a new rating is computed. The input to the calculation consists of

  1. The result of the game (win, lost, draw). The scores (e.g. 325 vs 207) are irrelevant.
  2. The rating of the players in the game just before the game ends
  3. The number of games each player has played.

Notes on Elo rating system for Spell It.

  1. A new player has an initial rating of 1200.
  2. A score difference of 400 represents a 10:1 chance of winning between the players. For example, suppose a player X with an Elo rating of 1700 plays against another player Y with an Elo rating of 1300. Then the chance that X will win is 10 times the chance that Y will win.
  3. Suppose player X is stronger than player Y. Suppose Z plays against one of these players. Winning over X yields a larger increase in Z’s rating than winning over Y. In contrast, losing to X results in a smaller decrease in Z’s rating than losing to Y.
  4. The Elo system has a parameter called the K-value. It specifies the maximum adjustment that can be made to the rating of a player after finishing a game. In the Elo system for Spell It, the rule for determining the exact value of the K-value is complicated. However, it can be said that a newer player has a higher K-value than a player who has played many games. For a new player, the K-value is 60.
  5. The Elo rating of all players is computed from data since Spell It was launched in December 2010 up to the present.

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