Matchmaking is the existing automated process in League of Legends that matches a player with, and against, other players.
The system estimates how good a player is based on win/loss ratio. It tries to matchmake players in a way that they have a 50% chance of winning , to ensure fairness and competition, which is one of the priorities of the system . However, this priority also becomes less and less important the longer a player takes to find a match, which is the reason of existence of the system .
Matchmaking takes into account premade numbers, so that groups are usually paired with other groups of similar size , especially due to groups being seen as having more advantages than pure solo players (communication and pre-knowledge of each other’s playstyles being factors) [625, 640]. This also determines the MMR of the team being an average of each member’s score, with some increase depending on how many premade members there are, which also involves the risk of getting tougher opponents .
Important is to note that MMR differs for each queue , which explains to some extent why players may occasionally meet high rankers in their normal matches.
In ranked, the very fact the matchmaking pool is small for high MMR players forced Riot to release and tune the “autofill” feature around July-August 2016, in order to decrease queue timers .
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