Statistical Plus/Minus (SPM) is an estimate of the player’s contribution to the team’s point differential per 100 possessions, using his box score stats as inputs. In another words, SPM can be considered as a way of estimating adjusted plus-minus from box score stats. Statistical plus minus puts a weight to basic box score stats like points, steals, blocks etc., in an effort to solve the lowest mean residuals (average errors).
The goal with the SPM technique is adding a more stable component to player’s basic stats as a counterbalance to the basic plus-minus. As a well-known fact, basic plus-minus can be wildly inconsistent for individual players from season to season or under different coaches in a season.
Dan Rosenbaum has outlined different ways of addressing adjusted plus-minus’ accuracy issues, by employing additional analyses based on box score stats.
In a lot of cases the statistical plus/minus ratings might be a more accurate predictor of future defensive performance. To characterize past defensive performance, adjusted plus-minus does a good job. A combination of the adjusted and statistical plus-minus will do a better job when we want to find out which players are going to be good or bad defenders in the future.
Statistical Plus/Minus is an “organic” way of deriving a linear-weights-style box score based formula from actual, real-life data.