Developer Insights: Hearthstone Battlegrounds Rating System Update
Lead Data Scientist on the Hearthstone team Tian shared in a Hearthstone Article that they are updating the Battlegrounds Rating System; Tian is here to share the science behind it.
What are the motivations behind updating the rating system?
We want our rating system to both provide a sense of progression through gameplay and create fair matchmaking. It’s very hard to achieve these two things simultaneously by using only one rating. Therefore, we designed a “dual-rating” system to address our intention. In the updated rating system, there are two ratings—external rating and internal rating:
- External rating: this rating is shown in your Battlegrounds lobby interface. The main purpose of this rating is to provide seasonal “progression”, as well as partially or fully reflect your actual skill.
- Internal rating: this rating is invisible to you. The main purpose of this rating is to help us estimate your actual skill at all times and thus create fair matchmaking.
What are the updates on the external rating (player facing)?
- Your external rating is reset to 0 at the start of each season (18.4 is the start of the first season).
- There is a rating “protection zone” at the start of the season—you are not able drop external rating if it is below or equal to 2000. Therefore, rating 0 is the minimum possible rating you would see on your user interface.
- There are rating “floors” between 2000 to 6000 for external rating. Your external rating does not drop below each “floor” once it’s above it. Currently there is a floor every 500 rating: 2000, 2500, 3000, 3500, 4000, 4500, 5000, 5500, 6000.
- Your maximum external rating gain per match is 300.
- After each game, if your external rating is below 6500, we will give you a very small value of positive rating gain, on top of your “actual” rating change, which could be either positive or negative. For example, in theory your rating would be added/subtracted by 50 after a game, but since we give you 3 progression points, in reality your rating value will be added/subtracted by 53/47. That small value is determined solely by your external rating—the smaller your external rating is, the bigger this “progression value” would be. Note that this number is so small that you won’t even feel it unless you play many games. When your external rating is above 6500, this value is set to 0.
What are the updates on the internal rating (under the hood)?
Your internal rating, which is invisible to you, will never get a seasonal reset like the external one.
The overall distribution of the internal rating should follow a bell-shaped curve – in mathematics terms, “normal distribution”. We might enforce this distribution by performing a procedure called “re-normalization” to your internal rating from time to time. Such adjustments are usually very small but necessary!
The internal rating is not limited to any rating protection zones or rating floors like the external one, and there are no progression factors being added on top of that. You and your opponents’ internal rating and variance are updated completely based on your game result.
How is matchmaking affected?
We will match players based on their internal rating only, as it is designed to always truly represent your actual skill. During the match building phase, we will try to select eight players with the closest internal rating to ensure the match is as fair as possible for everyone.
Meanwhile, external rating plays no role in the matchmaking process. Therefore, it is possible a player with 7000 external rating is in the same game as a player with 1000 external rating, if their internal rating values are close. If you see a player with 0 rating in a high level Battlegrounds game, it’s because they have a high internal rating and are likely just getting started with the Battlegrounds season!
How do external rating and internal rating interact with each other?
As everyone starts the season with 0 external rating, and matches are “fair” in terms of skill, generally we want players with higher internal ratings to receive more rating gain after each win, while keeping the amount of rating lost after each loss the same for everyone.
Specifically, when your pre-calculated rating gain is positive, it would be multiplied a factor called a “rate gain modifier”. This modifier is proportional to the discrepancy of your internal rating and external rating, when your internal rating is larger than your external rating. You can think of this as a “chasing” process, as the external rating is “chasing” the internal rating. If your external rating exceeds your internal rating, the chase stops.
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