How exactly do placement matches work?

[Kaawumba-1133]

@Kaawumba-1133

Players’ 10 placement matches will start with Blizzard’s best estimate of each player’s MMR for matchmaking purposes (15). A player’s initial SR will generally be close to his previous season ending SR, with a relatively ordinary adjustment based on 10 placement matches.

However, occasionally a player will have a large movement (24) in either direction. This happens on relatively inactive accounts (39), and can be explained by Scott’s statement: “Play a lot of games, it (MMR) gets more certain. Don’t play Overwatch for a while, it gets less certain. The more certain the matchmaker is about your MMR, the less your MMR will change in either direction based on a win or loss” (2). Essentially, if a player only does his placements, or slightly more than that, in a season, his MMR/SR is prone to much wider swings when he returns.

Because of reasonably high quality data for seasons 6, 7, 8, and 9 (24), it is possible to derive a formula: (New Season Starting SR) = (-173.7 +/- 20.8) + (1.0015 +/- .0059) * (Previous season Ending SR) + (37.94 +/- 2.23) * (Placement Wins). New Season Starting SR has a standard error of 181 SR (24), which implies a possible change in rating during placements of +/- 500 SR for inactive accounts. For active accounts, the max range is more like +/- 200 (from winning or losing all matches).

Decay does not persist through season transitions. That is, if a player decays from 4000 to 3000 before the season break, he will typically place around 4000 after the season break. Of course, if he is an inactive player (which decayed players often are), he may see an unexpectedly large swing in either direction.

Prior to season 6 (September 2017), players’ SR would be bumped down below their MMR at the beginning of the season, and they would earn it back with an SR buff over the course of 50 matches. This felt bad and was removed (15).

(2) Overwatch Forums The statement, “You go on a large win or loss streak, it gets less certain” is no longer valid, as win streak bonuses (and loss streak penalties) have been removed from the game.
(15) Developer Update | Upcoming Season 6 Changes | Overwatch - YouTube
(24) Season Placement Data - Google Sheets
(39) Season 9 Placements Analysis, High Variance Explained