Elo rating system: start value when players can join the game constantly
I've implemented an Elo rating system in a game. There is no limit for the number players. Players can join the game constantly so the number of players probably rises gradually.
How the Elo values are exactly calculated isn't important because of this fact: If team A beats team B then A's Elo win equals B's Elo loss.
Hence I've got a problem concerning the starting values for my rating system:
- Should I use the starting value "0" for every player? The sum of all Elo values would be constant. But since the number of players is increasing there would be some kind of Elo deflation, wouldn't it?
- Should I use any starting value greater than 0? In this case, the sum of all Elo values would constantly increase. So there could be an Elo inflation. The problem: Elo points lose value but the starting value keeps always the same.
What should I do? Can you help me? Thanks in advance!
Answer
You can start at zero and add a fudge factor to the displayed score to keep it above zero, or you can start at 1000 - they are the same thing. Yes, with the 1000 starting point you'll have an increasing number of total ELO points in the system but it will always be the same number per player on average - 1000. The starting value for Elo is always the current average. ELO is a zero sum game, points lost by player A are gained by player B.
When you set a starting point at 1000 what you are essentially saying is that the average player = 1000 pts. With a closed group of initial players (beta testers?) this is true, within that group average = 1000. But if the game is something you improve at with time then your closed group average player becomes highly skilled compared to someone who hasn't played.
Now when you assign a 1000 to a new player you are saying new average players = existing highly skilled average player. This is not true, they are likely to be much less skilled that your original closed group. So the new player loses points and your highly skilled players gain => inflation. What you would need to do is accurately assess the skill of new players and assign them a ranking that is more in keeping with their actual skill. This could be done be assigning them a "provisional ranking" for their first x games until you get a feel for their skill. When provisionally ranked only their ELO score would change, not those of the people they play. Once they join the real system the points they bring into the scored ELO would roughly equate to their actual skill and they wouldn't move up or down dramatically => no inflation or deflation.
In short: Provisional rankings