Saturday, July 26, 2014

Riot changed the matchmaking algorithm to address the blue side advantage - but also introduced a bug that place high ranking players on the purple side close to 70% of the time

Previously I made a bunch of posts describing the blue side advantage in League of Legends and how it is affected purely by camera angle. Apparently, purely because of how the camera is set up in the game, blue side wins 55% of the time. This was in response to several posts on reddit complaining about the issue.

In response to this, Riot seems to have changed something that dramatically reduced the blue side win rate from 55% to around 52%. Here's a plot of the day-by-day blue side win rate in North America Solo Queue around early June.

We see that the blue side win rate was around 55% until June 3rd to June 4th, where it sharply dipped to around 52%. This has been the case until the time this blog is written (July 24th).

How did this happen? What changed on June 4th that could have accounted for this? There was a patch on that day, but there was nothing to address the camera angle. As it turned out, Riot changed the matchmaking algorithm to put the stronger players on the purple side.

What evidence is there? Well, it turns out that after Riot's changes on June 4th, high elo players (i.e. Diamond and Challenger players) have a higher chance to play on purple side than on blue side. Prior to the change, a Challenger player has roughly 50% chance to play on blue and purple side. After Riot made the changes to put better players on the purple side, Challenger players now have ~58% chance to play on the purple side.

Making matters worse, the higher you are on ladder, the more likely you will be playing on the purple side. Take these players for example:

Vasilli, Sneaky (main and his smurf), Popstar Adrian, Meteos, and Mor

I have 340 games of these players on record since June 4th. Out of these 340 games, 68% of them had these top players playing on the purple side. I have 580 games of them prior to June 4th and they played on purple 49% of the time.

I think there are two consequences of this.

1. Riot essentially acknowledged that blue side has an advantage. Previously Riot was fairly mute on the blue side advantage, in particular to how it affected competitive play. This stealth change to matchmaking confirms that Riot does believe blue side has an advantage now. What consequences does this have for the competitive scene?
2. Riot most likely never had higher MMR teams on the purple side prior to June 4th, 2014. I often hear that Riot always put the slightly better players on the purple team. If this were the case, then prior to June 4th we should also see challenger players playing on purple more often - but unfortunately this was not the case and challenger players used to have 50% chance to play on blue or purple.

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