The basis of the Derbytron rankings is to order teams by their relative quality. Essentially, higher ranked teams should beat lower ranked teams. #1 should beat #2, #34 should beat #35, etc (assuming these games are played on a neutral floor). This isn't a system that will determine who is having the best season or who has the best history. So, how does it work?
My goal with this equation is to rate average teams as .5, better teams move closer to 1, and worse teams move closer to 0.
While combing through data and stats and graphs and more stuff, I began noticing that the most important games in determining how good a team really is are very close games and games against evenly ranked teams. So, those games have become more important in the ratings by adjusting their overall weight, up to 20/1. This essentially makes blowouts meaningless, unless the teams are closely ranked, as they should be.
The margin of the game is multiplied by a number and taken away if a loss or added if a win from the opponent's base rating (see below) and then averaged with the other games the team has played based on the games' weights. This number is the team's final rating and what determines the rankings.
Other Variables
Where was it played?
6 points are added to the away team's score and 6 points are taken away from the home team's score to determine home track advantage. In 2009, the home team won by an average of 16 points but with error and observation, this was too many points so I trimmed it down. If a game is close enough where the addition and subtraction of points changes the winning team, pre-determined numbers are substituted for each team's new game ratings. Neutral games are unaffected by this variable.
When was it played?
Recent games seem to be a better indicator for a team's strength so a higher weight is given to the most recent three games of the current season. A slightly lower weight is given to the next two and then a lower weight is given to any games played before that.
Games from the previous season are also counted but only after July 1. Not all games are used if teams played more than five games in the those six months. In that case, only the last five are counted (unless multiple games were played on the same weekend (nationals and regionals being the prime example)). These games carry much less weight than games from the current year.
Teams that haven't played anyone else suck.
Teams playing their first game are not counted. All of the previous averages that I've mentioned are determined without the current game as a factor. But, the averages of teams that have only played two games wouldn't be averages at all, they'd just be the ratings of the one other game they've played. So, for teams that have only played two games, both games are calculated into their average ratings.
What games are being used?
Currently, data is only being taken from WFTDA Sanctioned games which means only WFTDA teams are being counted in the ratings. Eventually, I would like to include every interleague team in the country including secondary travel teams and non-WFTDA teams. That is not possible at this time.
Base Ranking
50% - What happened in the game? That's important, right?
This is determined using the pythagorean expectation. Essentially, it's an equation that uses the score of a game to predict a team's winning percentage. Using this method will produce a number between 0 and 1. A close game will give a number close to .5 to both teams. A blowout will give a number approaching 1 to the winning team and a number approaching 0 to the losing team. This is what I call the vacuum game rating as this number only takes into account the score of the game and whether it was home or away but not the quality of the opponent.
25% - Who are you playing and what does their vacuum look like?
The vacuum rating is then combined with the opponent's average vacuum rating. This number gives a good indication of the quality of an opponent but it is not everything. A team could be very good but if they've only played the top 4 teams, they may have a very low vacuum rating. That is why it is only 25%.
25% - Who has your opponent played? Kind of important.
How has the opponent fared when compared to their competition? This is a number calculated by combining game ratings with opponent ratings and giving a more accurate description of the quality of the opponent.
Hopefully all of this makes sense. Any questions you have should be posted in the comments (don't be shy) and I'll reply and update the method description with more accurate information.
8.10.2009
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"Eventually, I would like to include every interleague team in the country including secondary travel teams and non-WFTDA teams. That is not possible at this time. I'm working on it, though."
ReplyDeleteThe data is available, as to how/whether to grant access to it would be someone else's call to make. There's always copy/paste.
It's tough to say where you'd draw the line to make the information remotely useful, I'd guess.
Even within WFTDA it's difficult to compare Long Island and Sin City or Big Easy and Gem City. Who'd win? None of them play the same teams enough to triangulate. At some point you get to the seventh degree of separation, so to speak.
With non-WFTDA teams even more so. I'd think you'd almost have to find a way to have it select teams from a particular state or region. Or even have it somehow allow you to select your own groups of states to compare. That'd take some doing.
And then there's the banked track leagues and the two geographical pockets of alt-rules derby that don't tend to have interleague bouts with outsiders.
You can compare the San Diego Derby Dolls with banked and flat track teams/leagues. You can't generally compare OSDA and Renegades teams to one another or anybody else outside of their niche. There's simply not enough playing outside their small circles.
There's a couple of leagues in Texas that play a game akin to flat track TXRD derby, and then there's the AZ Derby Dames (alt-rules flat track with few interleague bouts, but they're moving to banked track soon).