Posted by Neil Paine on November 20, 2009
Okay, it's time for the inaugural edition of the BBR Rankings, which will be coming to you every Friday from now until the end of the season. The idea behind these rankings is to rate a team's past accomplishments rather than predict what they'll do in the future -- sort of a retrodictive ranking rather than a predictive one. The Simple Rating System is the best predictor of future wins and losses because it focuses purely on point differential, filtering out the whims of lucky or unlucky performance in close games (which has been shown to have no bearing on future performance regardless of coach or players). However, it doesn't really give teams credit for wins and losses, which I feel like is the only real factor when you're looking at how impressive a team's season has been in the past.
To take a college football example, Oklahoma has a very impressive point differential this year against a very tough strength of schedule, and consequently their SRS is 5th-best in the country. Unfortunately for the Sooners, though, they also have lost four games. Would the Sooners likely win against most of the teams with fewer losses than they have? Yes, probably -- according to SRS they should be favored vs. everyone in the FBS except Texas, Florida, Alabama, and TCU. But no one in their right mind is going to rank OU ahead of undefeated Cincinnati and Boise State, or 1-loss Georgia Tech if we're talking about accomplishments, because the bottom line is that Oklahoma lost four games, and no amount of predictive advantage in the future is going to change that fact in the past.
All of this is to say that the premise of these rankings is to rate NBA teams on their past accomplishments rather than their likelihood of winning in the future. They're meant as a compliment to the SRS, and in fact, the basic structure of the ratings is the same as the SRS. Here's how they work: I calculated the average margin of victory for a winning team this year at home and on the road, and then averaged those together to determine the average point differential of a winning team on a neutral court (I didn't take a simple average of all winning teams because home teams are more likely to win by bigger margins, therefore skewing the value away from neutral). This is the baseline value that I give a team for winning (or deduct from them for losing). However, I also calculated the average MOV for teams at home this season, which is our HCA value. For teams that won at home, I gave them the avg. winner's MOV minus the HCA; for road winners, I gave them the winner's MOV plus the HCA; home losers received the avg. loser's MOV minus the HCA; and road losers got the loser's MOV plus the HCA. These were our static values for each team, crediting them only for wins and losses (and where each game took place). I then calculated the SRS normally (with HCA factored in), and those values were used for the SOS component of the BBR rankings. So, in effect, point differential does not matter when assessing the team's won-loss performance, to credit teams for "just winning" regardless of margin, but it does matter when assessing the strength of a team's opposition. I like this because there's no way to game the system by running up the score -- when you run up the score, you're actually only helping your opponents.
The end result is a ranking that takes into account strength of schedule in a legitimate, predictive way, but also credits a team for their own performance in the past. So, if you've managed to make it this far, here are the first BBR Rankings of the season:
|4||Los Angeles Lakers||9||3||0.750||2|
|6||Portland Trail Blazers||9||4||0.692||16|
|13||Oklahoma City Thunder||6||6||0.500||7|
|19||San Antonio Spurs||4||6||0.400||1|
|20||New Orleans Hornets||5||8||0.385||3|
|26||Los Angeles Clippers||4||9||0.308||23|
|27||Golden State Warriors||3||8||0.273||27|
|28||New York Knickerbockers||2||9||0.182||20|
|30||New Jersey Nets||0||12||0.000||21|