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Site map > Ratings
FIBS is a free site and exists purely for the fun of playing backgammon. However, as you win and lose matches of a fixed length your FIBS rating will be affected. In the overall scheme of things ratings are not that important, and FIBS also offers unlimited matches which are not rated.
If you win, your rating goes up. If you lose, it goes down.
If you beat someone who is better than you, you will get more points than beating someone who is worse. Similarly, if you lose to someone who is worse than you, you will lose more points than if you lost to a better player.
For example, a player rated 1500 plays a player rated 1925 in a seven point match - if he loses, he loses 2.28 points, but if he wins he will go up 8.31 points. How to work this all out is shown below.
Over time you will reach a rating where the points you gain for winning games is roughly equal to the points you drop for losing games - at this stage, your rating is broadly representative of your ability.
Your gain/loss is dependent on three things - difference in rating, the match length and your experience. For now, let's ignore experience to explain the thinking behind the system, but we will return to this right at the end.
The difference in rating can be converted to a theoretical winning probability. This is expressed as a number between 0 and 1 and is calculated with the following formula:
Winning prob. = 1-(1/(10^((YOU-HIM)*SQRT(ML)/2000)+1))
where YOU=your rating, HIM=his rating, ML=match length
Each match has a value in terms of points - this formula is much simpler:
Match value = 4*SQRT(ML)
The points you gain or lose depend on the result of the match:
If you win: Gain = Match value * (1 - winning prob.)
If you lose: Loss = Match value * (winning prob)
As an example, suppose a player rated 1500 plays a player rated 1925 in a seven point match - he has according to the formula a 0.2151 chance of winning. A seven point match is worth 10.58 points (square root of 7 multiplied by 4). If he loses, he loses 2.28 points (0.2151 * 10.58), but if he wins he will go up 8.31 points (0.7849 * 10.58).
Lastly experience - every time you play a match, regardless of result and score, your experience points are increased by the match length. To get players to their 'correct' level fairly quickly, the gains and losses of new players are multiplied by the following formula until they reach 400 experience points, after which the normal gain/loss is used:
New Gain/Loss = (Old Gain/Loss) * (5 - (experience points after the match/100)
So, if we look at the example above again, if the 1500 player is playing his first match against the 1925 player (an initiation of fire!), his gain/loss would be multiplied by 4.93 - so if he won that first match he would go up by 40.95 points!