How Accurate is SR?
On the forums there are generally two factions with respect to SR, those who think that SR is essentially luck and a meaningless value, and those who believe that SR is essentially correct. Neither of these factions have the correct approach. The correct question is to ask how accurate is SR? Clearly top 500 players are better than bottom 500 (see any bronze to GM series or watch low bronze play vs top play), so SR is not completely random, but how accurate is it really? For this discussion, I am assuming that we are talking about a player that plays to win every game, doesn’t share his account, and has played at least 100 competitive games, and continues to be an active player.
There are number of ways to approach this question. One is to start a completely new account, and then play 100 games on the new account and see how it performs compared to the old one. This shows that SR can vary by 1000 SR in extreme cases, and 500 in normal cases (27). There is some evidence that reroll experiments show less variance at higher ranks (28) likely because there are less random variables, such as smurfs, throwers, and inconsistent play.
Next, any player can see how his SR changes during a season. A range of 500 is completely normal here.
In addition, if an account needs a 55% win rate to maintain SR (11), and if win % changes slowly with rank, then it is expected that this will be an additional source of significant error in SR.
If a player gets noticeably better, it can take a long time to get to the correct rank. If a player maintains a 55% win rate, he will only go up approximately 220 SR/MMR every 100 games. Because there are twelve players in a match, one player’s contribution (and ability to carry the match) is limited unless he is playing at a vastly different skill level, so a 55% win rate for a player that is moderately under placed is to be expected.
Put this all together, and we can state that an active and motivated player’s SR is only accurate to +/- 250 SR in normal circumstances and +/- 500 SR in extreme cases. Of course, new players, barely active players, or players that actively break the system can be off by much more.