In 2015-16, our projections were based on a combination of
Real Plus-Minus (RPM), a system that rates each player primarily based on how his team performs when that player is
on or off the court, and
Box Plus/Minus (BPM), a system that rates players using traditional box-score statistics such as assists and steals. Last year, we switched to using BPM only. Why? There are a lot of things to like about BPM, including that it’s considerably more transparent than RPM, and it can be calculated going back several decades, making for easier historical comparisons.
But as a predictive tool, BPM does not appear to be as accurate as RPM. Instead, BPM has trouble picking up on factors such as defense and team cohesion. That led CARMELO to overrate teams such as the Minnesota Timberwolves and underrate more defensive teams such as the San Antonio Spurs last year. If we’d run the numbers using RPM instead of BPM in 2016-17, our projections would have been above-average again as compared with the projection systems that APBRmetrics tracks, we discovered.
All of this stuff gets complicated, and discussions can quickly devolve into alphabet soup. But for better or worse, the choice of metric matters quite a lot.
According to BPM, Russell Westbrook’s 2016-17 season was easily the greatest in NBA history.
According to RPM, he was only the ninth-best player in the league last season.
The upshot is that in the short run, we’re using a blend of two-thirds RPM and one-third BPM for this edition of the CARMELO projections. In the long run, we’re interested in developing our own plus-minus stat (but no promises about that quite yet).