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StarCraft 2’s Battle.net Leagues, Ladders, and Rankings Explained
Starcraft 2 has one of the most robust systems for ranking players of all skill levels and giving them fair games and a sense of progression. Blizzard doesn’t want to reveal too much detail about the way the rankings work as to prevent gaming of the system, and as such, a lot of speculation has caused a lot of misunderstanding among the player base. Still, we know that the ladder system is based around a few core principles and we have a lot of information that can be pieced together, so let’s examine the inner workings of the ladder system.
First, let’s tackle something a lot of people don’t know: the only thing that affects your ranking is winning or losing. Any good rating system for a competitive game must operate in this way. If it was based on how long the match takes, actions-per-minute (APM), or other factors, players could easily inflate those numbers to artificially increase their rating. Like ELO, the chess rating system on which Blizzard’s rating systems are based, each player has an associated number representing their skill level. In addition, there is an important second statistic called volatility that represents the algorithm’s confidence that your matchmaking rating (MMR) is accurate. This innovation allows players to be ranked very quickly and jump right into a range close to their skill. In other systems, like Warcraft III and even WoW’s arena system (to a degree), players must start at the bottom and work their way upward. The Starcraft 2 system prevents “n00b stomping” by creating new accounts and other abuses.
If you’ve tried the Starcraft 2 multiplayer, you are familiar with the process of acquiring a ranking. It asks you if you first want to play some practice games, which are played on a slower speed with “no rush” rules enforced. It is unclear if this data is incorporated into your early MMR, but I suspect that it is, as there is no drawback to starting players with an accurate rating. You then play 5 “placement” matches, where it chooses opponents from across the spectrum to try to get a general idea of your MMR. Keep in mind that it is only looking at win/loss data, and there are a lot of variables that go into deciding victory in a Starcraft 2 game. As such, with just five data points, the MMR it approximates for you is going to have a very high volatility.
Another key point to keep in mind going forward is that MMR is the sole factor in determining your opponents. League, rank and other factors have absolutely no hand. A high-ranked silver player could be playing a low ranked platinum player. If you have recently played an opponent, the system may shy away from matching you with them again, but this is just a theory I have carried over from Warcraft III’s matchmaking system.
With an MMR now associated with a new player, Battle.net will place players in a league. While your MMR is always hidden, the system seems to use only this number to place players into leagues. There are 5 leagues that players can be placed into, and in descending order of quality and approximate player base, they are:
Diamond (top 10%)
Platinum (10-25%)
Gold (25-45%)
Silver (45-70%)
Bronze (70-100%)
The system will never place a new player into the Diamond league after their placement matches.
Within each league are divisions. Each division is a small group of 100 people against whom members are ranked against. For example, one of my 2v2 teams is in the Gold Mutalisk November division. I can see all of the other players in this division, which gives me a ballpark idea of where I rank globally, but I do not play anyone in my division more or less than anyone else close to my MMR. The purpose of the division is simply to give players a sense of progression with a number that has less variance and more weight than a global ranking six digits long.
There are certain points at which Battle.net re-evaluates your league placement. The number of games or time-frame is not disclosed, but occasionally after a game, you will be placed into a new league. This can even occur after a loss. Players can be promoted or demoted to higher or lower leagues based on their performance. This is the only way to access the diamond league, as the system does not want 5 data points adding players into Diamond only to quickly remove them. Diamond status should be an “elite” league and carry some weight. During the initial rush of unranked players that occurred in the past week since release, the system has had very little data and, in my limited experience, low accuracy. My placement matches put me in the silver league, but after about 25 games, I’ve been promoted to gold and then to platinum, where I’m holding the top 10 in my division pretty well.
Now, how does your MMR change and how does this relate to points? We will explore this later this week with lots of number crunching.
Now that we have the basics down from the previous article, let’s look at how the MMR and rankings in StarCraft II.
One of the design goals of StarCraft II’s multiplayer is to match players with opponents such that they win 50% of their games on average. The system does this by matching players very close in MMR (matchmaking rating); the closer the MMRs, the better. Players identical in skill level should often have close and exciting games, a recipe for fun! The details of the search algorithm are not known, but it seems that it begins by searching for players in a a very small range around your MMR and slowly expands it (perhaps the “expanding search?”) (and probably in a logarithmic range as opposed to linear) as time searching progresses. Once players are found, they are put into a game.
MMR itself is never displayed and as such the details of its calculation are very hard to gleam. Assuming its similar to Elo, the system will increase a player’s MMR by more if they defeat a relatively more skilled opponent and decrease it by less if they lose to a more skilled opponent. The Elo-style math is a bit messy, but essentially the algorithm calculates a percentage chance that each player will win based on the players’ MMRs. The chance scales logarithmically with their differences; a 400 point rating difference might means A has a ten times greater chance of winning than B, eg, about a 90% chance to win. If a player wins, the system increases their MMR by the chance that they would lose times some system-wide constant, K, and does the opposite for their opponent. For example, if K is set to 12 and player A has a 90% chance of winning against B, and A does indeed win, his rating would increase by only .1*12=1.2, while B’s rating would decrease by the same. If B won, his rating would increase by .9*12=10.8 and A’s would decrease by the same amount. The system does not punish B very much for having to play A, but rewards him significantly if he does well.
Ideally, Battle.net would never match players this far apart in rankings. Additionally, the values for K and the exact logarithmic scaling numbers are unknown (though essentially unimportant to the behavior of the system). The system also almost certainly takes into account volatility in calculating MMR changes. A new player could swing hundreds of points in MMR when first playing. If you win all five of your placement matches, it will probably rate you very high and match you with very high rated players, but a couple losses will plummet you to a new “test” range until you are winning 50% of your games and your rating doesn’t change much.
This is all well and good, but we never see any of the numbers used for matchmaking, only for ranking. Rating refers strictly to MMR, but ranking determines your place within a division and has nothing to do with MMR. Bizarrely, the system matches your opponents in a different way than it ranks you. Let’s take a crack at rankings and points.
Within each division, players are ranked by how many points they have. Points are earned by winning matches. The number of points you earn from a win or lose on a loss is calculated by the following:
p = 12 + x + b
p is your calculated change in points. x is some integer from -12 to 12. This number is picked based on players comparative MMR. Finally, b is some amount of points from your bonus pool.
You can tell what x is going to be from the loading screen for a match, where the game tells you who is favored to win. The system calculates your opponents MMR if they were to beat you (their “projected rating”), then compares to your current rating. If it is nearly the same as your rating, the game is displayed as “even”, and x=0. If their rating is a little better , they are “slightly favored”, and x is 1, 2, or 3. If there is a big difference, then they are favored, and x is between 4 and 12, inclusive. The same mechanic works if you are favored, except that the numbers are negative. For example, if you are favored in a match and the system calculates the difference between your current rating and your opponents projected rating corresponds to x=8, then you would gain 4 points for a win and lose 20 points on a loss (excluding bonus pool).
The weird part about using the player’s rating and the player’s opponents projected rating to display who is favored in a match means that players can see different data on who is favored. If your rating has a high volatility, like when you first start playing on the ladder, your rating will drop more on losses, meaning that you will see a lot of opponents “slightly favored” or “favored” and have the opportunity to gain more points. This, combined with the bonus pool, makes the points system much more than a zero-sum game.
The bonus pool is a set of points that accumulates over time and on different events. It seems to generate at a constant rate over time, and new players seem to get the total number that would have accumulated for them had they started at the start of the season. If you win a game, then some number b of points is drained from your bonus pool and added to your point total. This number is up to 12+x if you have enough points available. For example, if your bonus pool has 4 points and you win an even game (x=0), you would gain 12+4=16 points for your win.
The league you are placed in uses your MMR, but your ranking does not. Does ranking actually “mean” anything? It seems more like smoke and mirrors; your ranking does not influence your match-ups, nor does it even necessarily correlate with your MMR. A top ranked platinum player by points could theoretically be on the verge of being demote gold league. Still, by and large, the leagues and rankings do give a sense of progression and a way to measure your skill and dedication against your peers without being labelled with a skill number. It’s a success for bringing competitive gaming to the masses, but hardcore players might just want to see their MMR.
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