ELO ratings for multiplayer game

Hi, been doing some research on ELO type of ratings (http://en.wikipedia.org/wiki/Elo_rating_system)
I want to implement it for multiplayer. I have been unable to find an exact formula for multiplayer on the web. But the math is pretty basic. I could just use another set of eyes on this to make sure it is sensible. Note; this is for a dominos game (no team play). For team -based multiplayer, ELO is not as appropriate.

What I've found on the web, here is the formula
R' = R + K ( Sa - Ea )
where
R' = player A new rating
R = player A current rating
K = KValue (e.g. commonly used 10, 15 or 25 for a provisional player)
Sa = actual score (e.g. 1.0 win, 0.5 tie, 0.0 loss)
Ea = expected score (from formula below)

Ea for player B = 1 / (1 + 10 ^ ((Ra - Rb) / 400 ))
Ea for player A = 1 / (1 + 10 ^ ((Rb - Ra) / 400 ))
where
Ra = player A rating
Rb = player B rating

OK so the part I'm not sure about, is how to expand the R' formula for the multiplayer case. It seems like one would repeat the term K(Sa-Ea) for each sub-match( a vs b, a vs. c, a vs d , in a 4 player match)

R' = R + K ( Sa - Ea1 ) + K ( Sa - Ea2 ) ... + K ( Sa - EaN )

I've just finished implementing this in C# and writing some tests now. Any comments or thoughts would be appreciated!

edit:
also here is a cool site illustrating what I'm trying to do. they don't show the formula though
http://elo.divergentinformatics.com/

 

If anyone cares to take a look and/or use this code you are welcome to it. I think this is a valid multiplayer usage of ELO algorithm but would welcome feedback. Seems to work great. I noticed something strange however, when calculating a Tie using 0.5 for the outcome. When you tie against a better rated player, your rating goes down and their rating goes up. Doesn't seem fair. Seems like the opposite should be true. Maybe for tie games I just won't calculate new ratings

edit: I found a bug in my code, I will add a new post below with update code and explanation.

 

Sorry for resurrecting this thread. The above website is mine. You should not be seeing the lower rating go down in a tie. Your expectation is correct ... the higher one should go down and the lower one should go up. I haven't looked at your code yet but I will compare it mine and see where the differences lie. Also the way my website calculates the score is by treating each opponent vs opponent as a simultaneous individual match and then summing up the results and applying it to the score. I have a breakdown of what is happening on the results page.

 

Hi rpo, thanks so much for the feedback confirming that-- I will post any future revisions here too.

 

Hi mindlube, did you ever found the way to solve the tie issue you were having? If so can you please post the fix to the formula?

Thanks!

 

Hi all! Thanks for your interest. I found the bug in my code. In the Ea calculation, there is a subtraction term and the order of player vs. opponent is critical. It was just plain wrong before, and was most obviously bogus in the case of tie games. I clarified the formulas above, am attaching my updated .cs files here, and I also have done a quick spot check against http://elo.divergentinformatics.com/ and it is correct.

 

Thanks for sharing and answering mindlube, you rock!

 

No problem! Glad you are finding it useful. My code is sloppy though- I just noticed that the sum is being calculated twice. But it is at least accurate now.

 

What would setting a player as provisional be for?

 

hello have you solved all problem which arise in this elo rating system??

i need such type or rating system fro my multiplayer game...

 

@mindlube If we use your formula, the K-factor will not correct anymore. Ex: if there are 5 players, the first place player can win up to K * 4 Elo rating.

 

Sorry for late post you need the multinomial version of the probability of win. We did this for Reiner Knizia's Samurai for multiplayer ELO.

The formula you need is the Softmax version of the formula for expected win:

Ea = Za / Sum(Zi, i = a, b, c, ...) for players a, b, c, ...
where Za = 10^(B * Ra) and for standard ELO B = 1/400

then just plug that back into your original formula for updating ELO rating's for each player:

Ra' = Ra + K * (Sa - Ea)

Best regards,
Gordon