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ELO score

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Bifurcations
By Bifurcations | Mar 4 2016 5:40 AM
I realise this has probably been asked loads before but how are ELO scores calculated?
admin
By admin | Mar 4 2016 9:19 AM
Bifurcations:
This is all based on Prof. Elo's original equation. Elo was a Hungarian statistician and chess player. The same system is used for chess or tennis or debating. It measures statistical probability of beating somebody else.

Everyone's Elo starts at 1500. Then, on the basis of results (1 for a win, 0 for a loss, half for a draw) it adjusts the Elo score of both sides.

Here is the actual PHP code used to calculate Elo changes on this site:
function elochange($myelo, $theirelo, $result) {
$expectedoutcome = 1 - 1/(1+pow(10,($myelo-$theirelo)/400));
$returnvalue = $myelo + 40 * ($result - $expectedoutcome);
if($result==1 && $returnvalue($myelo-5)) $returnvalue=$myelo-5;
return intval($returnvalue);
}


What this does:
Declares a function based on your elo, the other side's elo, and the result of the debate.
Calculates the "expected outcome", an index from 1-0 based on how likely it is you would win. 400 is the critical value (called "k") which controls how sensitive the function is to differences in Elo scores.
Calculates the new Elo score. As you can see this is based on the difference between the result and the expected outcome. I use a range of 40 which tends to work quite well.
Then finally I put in a condition that winners must always gain at least 5 Elo points, and losers always lose at least 5, regardless of the rating difference.
The very last line rounds the score to the nearest integer (ie removes any fractional part).

The simplest example of this is if 2 new debaters battle. The expected outcome is 0.5, because since they are exactly evenly matched in Elo scores, the system presumes they will draw. One will win however, and get 0.5*40 Elo points = 20, and end up with 1520 Elo.

If a debater beats me, that would be relatively surprising, since I have the highest Elo score on the site. So then they get more Elo points if they beat me. If they lose to me, however, that would not be too statistically surprising, so they don't lose as many Elo points.

Hope this makes sense :)
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admin
By admin | Mar 4 2016 9:20 AM
admin: Also, there are no draws on edeb8, and I use a more complex system for calculating 4-way debates, but this is basically it.
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Bifurcations
By Bifurcations | Mar 4 2016 10:05 AM
@admin: Yeah this makes perfect sense :) thank you
Bi0Hazard
By Bi0Hazard | May 6 2016 7:02 PM
admin: Has anyone ever beat you?
admin
By admin | May 6 2016 7:08 PM
Bi0Hazard: Sure. Been about a year since it's happened in online debates though. When I started out online (about 6ish years ago) I used to lose a good 50% of the time. It comes with experience a bit.

Of course in real life debates, I haven't really debated since 2012, so who knows. XD
I'm the main developer for the site. If you have any problems, ideas, questions or concerns please send me a message.
Let's revive the forums!