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

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

Notes on Elo rating system for Spell It.

- A new player has an initial rating of 1200.
- 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.
- 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.
- 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.
- 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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