Since the quality performance decay (QPD) is now working as intended, I figured I would do a quick write-up with regards to its purpose.
In short, the QPD is meant to gradually decrease the rating of a fighter who has not necessarily been a victim of losing, or inactivity, but one who has not recently had a performance that’s indicative of a fighter of that rating. It is important to note that a quality performance is relative to the fighter’s rating.
Without digging too deep into the decay, in order to register a quality performance, a fighter must beat an opponent at least 1/3* of their pre-bout rating, draw against an opponent at least 1/2* of their pre-bout rating, or lose a split/majority decision against an opponent at least 2/3* of their rating. *Values used are approximate. Note: UFC/PRIDE title bouts have/had special considerations.
Please remember, that even if a fighter draws or loses a close decision to a lower rated fighter and registers a quality performance, they will still lose rating points as a result of the bout’s outcome.
If a fighter has went 360 days without a quality performance, they go into a mode of QPD. This decay is not nearly as strong as the inactivity decay, but is a bit less forgiving than a fighter coming back from inactivity as an inactive fighter can (in some scenarios) redeem a bit of their lost rating by performing well against a quality opponent in their comeback bout. The reason for that forgiveness is that we can gauge a fighter in QPD, but not one who is inactive.
Once a fighter in this QPD registers a quality performance, they are taken off of the decay mode. It is also important to note that since a quality performance is relative, as their rating slips, the requirements on breaking free from the decay are lessened.
The QPD is one of the main advantages of the rating system. It is gradual, and no fighter is immune. Many ranking sites may forget about certain fighters who have been bottom-feeding, or simply leave them at their position due to prestige. They may also over- or under-correct these fighters. This is not the case here. The decay treats everyone equally, as it is gradual, and proportionate.
Notable fighters currently in the QPD are: Fedor Emelianenko, Josh Barnett, Mirko Filipovic, Wanderlei Silva, Matt Lindland, Ricardo Arona, Antonio Rogerio Nogueira, Frank Trigg, Takanori Gomi, and quite a few others.