"Whenever it wants to make a game, it calculates an expected win percentage for each team based on the [Skill Rating] of all the players. If one of the teams doesn’t have at least a predicted 40% win rate, then we simply don’t create the game."
– Scott Mercer, principal designer at Blizzard, writing about how Overwatch's matchmaking works.
Blizzard's Scott Mercer recently posted a fairly in-depth post about Overwatch matchmaking on the company's forums, and it makes for intriguing reading if you're at all curious about matchmaking tech.
Even if you're not, Mercer included some interesting stats in his explanation of how everything that works that shed some light on the state of competitive Overwatch.
For example, between February 1st and May 28th just 16 percent of all competitive matches were filled with solo players. However, in the same period teams of all solo players were most likely (73 percent) to be matched up against other all-solo teams, and in such match-ups the win rate for either side was roughly 50 percent.
"The matchmaker was designed to try and create games with equivalent-sized groups, especially for solo players, and these statistics show that it’s doing a pretty decent job," wrote Mercer. "When the matchmaker does compromise, it’s usually during off-hours when there aren’t as many available players, as well as at the lowest and highest extremes of SR."
However, Mercer goes on to note that the matchmaker won't compromise past 20 percent (i.e. it won't set up a match if a team has less than a 40 percent predicted chance of winning), a change made earlier this year to try and prevent one-sided games. For further details on the change (as well as more stats) check out the full forum post.