EPT
http://imgur.com/a/vaLDF#0
Why Does MMR Decay Exist?
As some might recall, many players actually requested that some sort of decay mechanism be implemented because they didn't feel they could compete at their normal level after returning from a long break. For example:
http://eu.battle.net/sc2/en/forum/topic/2868804407
http://us.battle.net/sc2/en/forum/topic/7179568535
Blizzard apparently agreed, and now MMR will decay if users have not played games for a certain period of time.
How Much Does MMR Decay Due to Inactivity?
Korona is still looking through MMR-Stats records, but there seems to be an upper limit of about 310 or so rating which is a little larger than the span of most leagues. If you're very inactive, you could drop one league (or, if you're already near the bottom of one league, up to two leagues) upon returning. Note that if you are inactive in a season and return in the same season, you will remain in the same league because you can no longer be demoted mid-season, but you will face lower-league opponents because MMR transcends league boundaries. This means that if you have experienced MMR decay mid-season, your league icon no longer reflects your current skill level.
Note that so far, we have not discovered any examples of players dropping more than the 310-ish value. However, we do not believe that this value applies evenly to inactive players, meaning the amount of decay is probably gradual beginning at some point and increasing up to this limit.
Also note that the existing MMR lookup rule applies: if you are inactive for an entire season (that is, you stop playing in Season n, you play no games--ranked or unranked--during Season n+1, you come back for Season n+2), your MMR is wiped clean.
What Impact Does MMR Decay Have on the Overall Ladder?
On an individual basis, decay is a fine concept. You come back from a break, you're a little rusty, so it's fair to match you against people who are a little weaker. On a macro scale, it depends on the overall activity level of the ladder.
I hope to update this post soon with inactivity filters included, if I can get a handle on them.
How Frequently Must I Keep Playing to Prevent Decay?
Decay begins to take effect between 2 weeks and 4 weeks of uninterrupted inactivity. We believe the effect to be linear, from a decay of 0 at 2 weeks to about 310 at 4 weeks. The below image illustrates.
![[image loading]](http://i.imgur.com/8TT5AnH.png)
The red line is the decay rate. x is time in days and y is decay amount. The decay hard caps at the blue and green lines.
Does MMR Decay Result in Ladder Deflation?
Based on the current activity levels of the ladder as a whole, it appears that MMR decay does have a deflationary effect. As noted, if over half the Platinum players go inactive and get decayed down to Gold, as long as their skills deteriorated at similar rates, they would be evenly matched against each other which means their ratings would not increase. However, against legitimately weaker true-Gold players, they would still win, which would effectively push the Gold players further down. As long as the inactive members constitute a majority of the player base, system-wide deflation is inevitable.
Is MMR Decay Overtuned?
It is arguable that the current upper limit of MMR decay is too aggressive. It's jarring for players to suddenly find that they are starting a new season in a lower league despite feeling that they were competing effectively in their higher league, only because they didn't play enough games. On average, players earn 16 MMR for a win against an opponent of equal strength. A decay-driven rating drop of 300 means that it would take nearly 20 wins over losses to return to a player's previous strength. Assuming you never lost, it would take you 20 games to overcome. However, according to my research, only 5,928, or 17.02%, of Bronze players played more than 20 games in Season 4, compared to 37.83% of Silver, 58.20% of Gold, 69.20% of Platinum, 77.59% of Diamond, and 85.70% of Master players. Overall, 52.12% of players will not play enough games to offset the maximum amount of MMR decay over the course of a season.
Notes
Decay triggers only after an uninterrupted inactivity period . Korona should have better visibility on this as time passes, but it shouldn't affect the integrity of the majority of information presented here.
+ Show Spoiler +
Incorrect/Legacy Information
The data captured reflects the Americas server. For Bronze league, 3,261 of 34,812 players (9.3%) had bonus pools below 1 week's worth by the end of last season. 3,691 (10.6%) had bonus pools below 2 weeks' worth. 4,165 (11.9%) had bonus pools below 3 weeks' worth. 4,718 (13.5%) had bonus pools below 4 weeks' worth. 5,598 (16%) had bonus pools below 5 weeks' worth. 7,775 (22.3%) had bonus pools below 6 weeks' worth.
For Silver it breaks down like this:
<1 week's worth of bonus pool by season end: 11.91%
<2 weeks: 14.9%
<3 weeks: 18.4%
<4 weeks: 22.8%
<5 weeks: 29.1%
<6 weeks: 40% of the Silver population
Here's the full breakdown if you're curious:
![[image loading]](http://i.imgur.com/igMs9eR.png)
Now, 6 weeks is pretty generous when you're talking about someone being an active player, so it's a pretty safe bet that the actual activity metric is below this. The fact that only 22% of Bronze players meet this criteria means that if you're playing at a Bronze level, 22% of the time your random opponent may be actual Bronze-level, while 60% or more of the time it may be an inactive Silver.
It's more common for higher-level players to keep up in activity. On average, around half of the Master and Diamond populations kept their bonus pools low. However, that's still a 50-50 shot that you're playing someone who experienced MMR decay versus someone proven to be at that level.
The data captured reflects the Americas server. For Bronze league, 3,261 of 34,812 players (9.3%) had bonus pools below 1 week's worth by the end of last season. 3,691 (10.6%) had bonus pools below 2 weeks' worth. 4,165 (11.9%) had bonus pools below 3 weeks' worth. 4,718 (13.5%) had bonus pools below 4 weeks' worth. 5,598 (16%) had bonus pools below 5 weeks' worth. 7,775 (22.3%) had bonus pools below 6 weeks' worth.
For Silver it breaks down like this:
<1 week's worth of bonus pool by season end: 11.91%
<2 weeks: 14.9%
<3 weeks: 18.4%
<4 weeks: 22.8%
<5 weeks: 29.1%
<6 weeks: 40% of the Silver population
Here's the full breakdown if you're curious:
Now, 6 weeks is pretty generous when you're talking about someone being an active player, so it's a pretty safe bet that the actual activity metric is below this. The fact that only 22% of Bronze players meet this criteria means that if you're playing at a Bronze level, 22% of the time your random opponent may be actual Bronze-level, while 60% or more of the time it may be an inactive Silver.
It's more common for higher-level players to keep up in activity. On average, around half of the Master and Diamond populations kept their bonus pools low. However, that's still a 50-50 shot that you're playing someone who experienced MMR decay versus someone proven to be at that level.
