For comparison the world golf rankings drop results completely after 2 years.
I know 10% doesn't contribute much to the ranking, but this still seems too long given how much things can change in 10 years. Mike, have you done any testing on optimal
dropout rates for old match results?
One problem with dropping results after two years is then you don't know anything about people that stop competing for extended periods of time and then come back.
This happens a lot in sports, but probably even much more so in pool than in many others. So when you quit playing for say 6 six years and then come back, yes you aren't going to be as good as you were six years ago at first, but you aren't going to suck either, plus your game is likely to improve very rapidly. And since FargoRate so heavily weights your most recent results, it quickly "overrules" your old data and very rapidly adjusts you rating to be very close to your true current abilities. There is a strong argument to be made that having rating info on someone, even if it is slightly off right at first for people that haven't played for an extended period of time, is loads better than having no info at all.
Another benefit to counting results that go back so many years is that it allows you to get a larger sample size of people's play. If everyone was competing once a week, and if all tournaments and leagues reported their results to FargoRate, then you would probably have enough info for super accurate ratings even if you only went back a year, but neither of those things are the reality. Most tournaments and leagues still aren't reporting their results to FargoRate and so FargoRate is having to rely on the results from just a small percentage of them. Probably an even bigger factor is that many players just don't compete regularly. Many just compete a time or two a year on average if that, and so you just don't have enough info for accurate assessments on them unless you collect information over a longer period of time. And there are many others that tend to compete in spurts. They may compete fairly regularly for a little while, and then they take one, two, three, or more years off from competing or playing, and so again you often just wouldn't have much info on these people much of the time if you only went back a couple of years.
All things considered, it seems like it makes the most sense to count results that go back quite a number of years, but weigh the more recent results much more heavily, thereby allowing you to get the best compromise between how current and accurate your ratings are, and the number of people you can provide pretty accurate ratings for. Essentially you could have super accurate ratings for a very small portion of players, or really accurate ratings for most of the players, and which of the two is best seems pretty clear especially when you consider that being able to accurately rate and be used by as many players as possible around the world was one of the main goals of FargoRate to begin with--one system that everybody can use regardless of location.