Originally Posted by Yegg
Yes, there is a system in place to temporarily reduce Elo when someone hasn't played for awhile. After they return and start playing games, the decayed Elo is given back to them on top of whatever Elo they gained or lost normally.
This hasn't always been the case though, so your breaks might have been before that system was implemented.
Are you sure that's a good idea? Since we don't care about the actual numbers a skewed normal ELO distribution shouldn't matter as much. So long as the system creates a partial order our matchmaking should not have significant problems in the long term.
When we give people back their ELO instead of making them earn it, in addition to any ELO they lost or gained, we are introducing even more variation into the system. We have higher variance right after they come back[because they're rusty and we are uncertain of the change in skill level] but higher variance in games after they have regained their skills because now we put them at an ELO that isn't the result of playing games against opponents in that area.
It seems like it would make a lot more sense to give the normal ELO reduction and then to let adjustments occur faster [like when you first start playing ranked] since what we are essentially dealing with is an increase in uncertainty and/or a faster skill adjustment time.
I.E. A person who leaves the game and comes back will suffer some amount of lost skills. This amount is testable in aggregate[i think] such that we can model the skill level of a player as E1=E0+Z(t) +e. That is New skill = old skill plus a time factor plus an error(which may also be a function of time but is left out for simplicity).
Since we have uncertainty surrounding their old skill their new skill level is even more uncertain even if we can make a best guess.
In addition we believe that they will regain their skills at some pace. We might generalize this to compare it to the skill advancement of a normal player. I.E. why we have a minimum amount of ELO loss/gain. Such we can model coming back to the game after being away as a player with a larger amount of uncertainty in their skill level.
It seems like this would produce overall better results in terms of reducing the number of "swingy" games than returning their lost ELO to them on top of what they had already won or lost.