Rating Differences

Sherwood, Russell  Tuesday, June 16, 2020

In the postbag (well the electronic one anyway!) I have been asked several questions about the ICCF rating system.  One wide misunderstanding  is about rating points being gained or lost playing against lower rated opposition. 

Looking into it two factors have the most underlying effect on the result – your k value and the difference in rating.

The k value is defined as part of the rating equation:

 

I put a K calculator into the spreadsheet to make life easier, but we can see from the above the k value gets smaller as your rating and number of games completed increase.

From looking at the formula we can see that the k value starts out around 25 and then trends down to around 10 for the majority of players – or 20 if your rating is below 2000. Where do you find your k value – either on the spreadsheet or in your rating forecast on ICCF.com

Once we know this we can determine that the number of rating points lost or gained in a game is calculated from your actual score in the game – your expect score multiplied by your k value.

Your actual score can only take one of three values 0,0.5 or 1. 

Your expected score comes from the difference between you and your opponents rating, by the formula:

Hmmmm…..

In practice this means

Rating difference

Diff%
2552%
5054%
10059%
15063%
20067%
25071%
30075%

 

By a very long and painful journey we come to a number of generalizations, which have been the questions I have been asked:

  1. You have to be rated more than 100 elo more than your opponent to lose more than 1 point from a draw (for most people with a K factor around the 10 mark)
  2. In a two game match (Friendlies in particular), this drops to 50 elo for a “double draw” costing more than 1  point
  3. If the difference in elo is less than 300, than 1.5/2 will be a net rating gain for the higher rated player 
  4. Once the gap is more than 300 elo then 2/2 is required to not lose rating points.

 

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Rating system

Updated Tuesday, June 16, 2020 by Russell Sherwood

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