The next big thing: Real Plus-Minus

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?

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?

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, opposing player and coach.

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.

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