The next big thing: real plus-minus
Basketball's two-way stat tells us how players perform on both ends of the court
Bulls coach Tom Thibodeau recently made the case for Taj Gibson as the NBA's Sixth Man of the Year. "I think that the biggest thing for him is what he has contributed to us winning," said Thibs. "The things that he does for us are all team-oriented. He plays great defense, challenges shots, guards everybody, runs the floor hard, sets great screens, does his job."
So Gibson helps his team win by doing all the little things that never show up in a box score. But how can we measure the value of running the floor hard, or making smart defensive rotations, or setting great screens?
And how can we tell if Gibson has been more valuable to his team than, let's say, Clippers sixth man Jamal Crawford, who has scored more points than Gibson (18.6 versus 13.2 PPG) on a higher true shooting percentage (55.7 versus 52.8) while logging heavier minutes (30.3 versus 28.8 MPG) for a better team in a tougher conference?
Today we're introducing an advanced metric that can help us out: real plus-minus (RPM).
What is real plus-minus?
ORIGINS OF RPM
Real plus-minus (RPM) was developed by Jeremias Engelmann, formerly of the Phoenix Suns, in consultation with Steve Ilardi, University of Kansas psychology professor and former NBA consultant.
It follows the development of adjusted plus-minus (APM) by several analysts and regularized adjusted plus-minus (RAPM) by Joe Sill.
RPM reflects enhancements to RAPM by Engelmann, among them the use of Bayesian priors, aging curves, score of the game and extensive out-of-sample testing to improve RPM's predictive accuracy.
As the name suggests, real plus-minus shares a family resemblance with the +/- stat in the box score, which merely registers the net change in score (plus or minus) while each player is on the court.
RPM is inspired by the same underlying +/- logic: If a team outscores its opponents when a player is on the court, he's probably doing something to help his team, whether or not he's putting up big numbers.
But the familiar +/- stat has a serious flaw: Each player's rating is heavily influenced by the play of his on-court teammates.
For example, in the basic +/- numbers, Thunder backup point guard Reggie Jackson is ranked 27th in the league. But he's also spent the majority of his minutes playing alongside Kevin Durant, the league's likely MVP. What we really want to know is how much of Jackson's elite rating is attributable to his own play, and basic +/- simply can't tell us.
But real plus-minus can.
Where does real plus-minus come from?
WHY "REAL" PLUS-MINUS?
"Real" comes from economics and such terminology as real income, real interest rate and real value. Instead of "nominal" value, for instance, economists prefer to talk about real value, which is adjusted for inflation.
Likewise, real plus-minus includes important adjustments to raw stats.
Drawing on advanced statistical modeling techniques (and the analytical wizardry of RPM developer Jeremias Engelmann, formerly of the Phoenix Suns), the metric isolates the unique plus-minus impact of each NBA player by adjusting for the effects of each teammate and opposing player.
The RPM model sifts through more than 230,000 possessions each NBA season to tease apart the "real" plus-minus effects attributable to each player, employing techniques similar to those used by scientific researchers when they need to model the effects of numerous variables at the same time.
RPM estimates how many points each player adds or subtracts, on average, to his team's net scoring margin for each 100 possessions played. The RPM model also yields separate ratings for the player's impact on both ends of the court: offensive RPM (ORPM) and defensive RPM (DRPM).
So, Gibson or Crawford?
Let's return to the previous comparison of Taj Gibson and Jamal Crawford. The chart shows their respective RPM ratings.
REAL PLUS-MINUS (2013-14)
Crawford has the higher offensive RPM (+1.53 versus +0.70), but the advantage turns out to be surprisingly small, given Crawford's scoring talents. It's worth only about one additional point per game.
And on the other end, the players are miles apart. In fact, Gibson's defensive impact (+3.42 DRPM) is large enough to give him a truly elite overall RPM rating (+4.12), placing him in the company of All-Stars like James Harden (+3.44 RPM), Chris Bosh (+3.75), Joakim Noah (+3.89), Russell Westbrook (+4.09) and Dwight Howard (+3.99).
In contrast, Crawford appears to give away a bit more on defense (-1.96 DRPM) than he contributes on offense, with an overall RPM impact (-0.43) that's slightly below the league average.
The Durant factor
RPM can also shed light on our question about the true impact of third-year guard Reggie Jackson, as the table shows.
REAL PLUS-MINUS (2013-14)
RPM accounts for the contributions of OKC stars Durant, Russell Westbrook and Serge Ibaka (as well as Jackson's other teammates).
After doing so, RPM indicates Jackson has had a moderately positive impact this season on both sides of the ball. His total RPM rating (+1.49), while far below that of his more heralded teammates, is consistent with the play of a quality NBA starter, and it's higher than that of more established guards such as Lance Stephenson, Kevin Martin and Wesley Matthews.
The King of RPM?
Let's close with a quick look at the game's greatest all-around player, LeBron James.
LeBron has been accused of coasting at times this season, and RPM reveals where the charge may have merit.
REAL PLUS-MINUS (2013-14)
While LeBron was aptly named to the NBA All-Defensive Team last season, his defensive impact this season has been surprisingly mediocre, at least as measured by RPM (-0.21 DRPM). His rating suggests James has not consistently given his best effort this season on the defensive end.
Does the same hold true on offense? No. Offensively LeBron has been as dominant as ever, and his +8.32 ORPM leads the league by a comfortable margin. This means that despite his less-than-stellar defensive contribution, LeBron's total RPM (+8.11) still rates as the NBA's highest this season.
Of course, that doesn't necessarily mean he should be regarded as the league's MVP. There's a certain prolific scorer in OKC who has also put up a stellar RPM number this season, and he's done so while playing more possessions than LeBron.
Later this week we'll look at how RPM can be used to estimate the number of wins each player adds to his team. In other words, we'll introduce a new lens through which to view this season's MVP race. (Spoiler alert: It's close. Very close.)
RPM stats are provided by Jeremias Engelmann in consultation with Steve Ilardi. RPM is based on Engelmann's xRAPM (Regularized Adjusted Plus-Minus). Play-by-play data provided by Basketball-Reference.com.
Also see: Check out Kevin Pelton's RPM All-Stars
MORE NBA HEADLINES
- LeBron: Cavs 'very fragile' amid 4-game skid
- Cavs collapse vs. Raptors, lose 4th straight
- Davis' career-high 43 push Pelicans by Jazz
- Rockets' Howard sits following PRP therapy