The purpose of the Heavyweight Boxing Top 100 computer rankings is to offer a purely unbiased, non-political alternative to the belt organizations. It's a computer ranking only in the sense that it would take a lot of time to do the simple calculations needed to formulate the ratings; it more accurately could be called an 'algorithmic' ranking system. The complexity of the boxing ratings comes from the fact that it uses 120+ years of data in the fight database.
The fight database is based on Michael G. Paul's boxing web site, All-Time Heavyweight Ratings, (now archives at Archive.org after Geocities' demise) and of course, the Boxing Record Archive. Between these two great sources I've compiled a database of fights that goes back over 120 years. Mr. Paul's site (which he last updated in late 2005) has its own rating system, and BoxRec.com updates their own ratings daily. Both are great sites that come highly recommended and I owe a debt of gratitude to both.
A fighter's rated record counts only the fighter's major fights. We keep a master list of heavyweight fighters who are considered 'rated' fighters; when a new fighter defeats one of these fighters, he is deemed a 'rated' fighter and added to the list. Any wins against non-rated fighters are ignored for purposes of these ratings. At any given time there are around 300 active 'rated' fighters in the heavyweight division.
The rating system is based solely on winning and losing fights. It's a "reward" system rather than a "predictive" system—that is, it doesn't try to determine which fighter would win a contest between two fighters, but rather looks at a result, after the fact, and gives points for success in the ring and deducts points for losing. Whether a fighter wins by KO in the 1st, controversial split decision in 12, or by his opponent's disqualification, the end result is the same. Draws are counted as half a win and half a loss, essentially.
The results are meant to be strongly 'lineal' in nature, meaning that when a fighter beats someone, he will be ranked above the fighter he beat, regardless of where they were previously ranked or what their ratings were beforehand. In this way it behaves in the same way the Lineal champion is determined: the man who beat the man becomes the man. However a fighter might beat the champion, but not become the champion. Nine times out of ten he will, but if he started ranked very low, for example, and/or the champ was just slightly ahead of the #2 fighter, the #2 fighter might move into the #1 spot. In all cases, though, the old champ would be ranked below the fighter who just beat him.
Fighters are never arbitrarily removed from the list. If a fighter retires or even dies, he remains on the list until inactivity reduces his rating down to zero. This takes place over a 5 year span: a fighter can be inactive (no wins or draws against 'rated' fighters) for a year before any penalties begin, then over the next four years the fighter's rating will gradually sink to 0. Any win or draw against a rated fighter halts this process and the calendar starts over, but any damage to a fighter's rating remains.
Also, boxers who lose five fights in a row are removed from the system and are no longer considered "rated." When that happens a fighter must "re-register" himself by beating (or drawing with) a rated fighter. This prevents a fighter with 20 losses in a row from "registering" a bunch of "bums" and crowding up the fights list.
At various times we may add more historical fights to the list that have been missed, or remove fights that are in error. This will sometimes have an effect on the current monthly ratings, but it's pretty rare and unavoidable as we try to keep a complete set of the fights we want in the database.