Plus-minus and net rating, explained: measuring impact beyond the box score
What plus-minus really tracks, why raw plus-minus is noisy, how net rating puts it on a per-100 scale, the leap to on/off and adjusted versions, and how to read them.
The basic idea
Plus-minus asks a simple question the box score never does: while this player was on the court, did the team outscore the opponent or not? If the team is +8 during a player's minutes, his plus-minus for that stretch is +8. It tries to capture the things points and rebounds miss — a screen that frees a shooter, a rotation that kills a drive, the glue that does not show up in the traditional stats.
Why raw plus-minus is noisy
Single-game plus-minus is easily misled. A bench player can post +15 simply because he happened to share the floor with the starters against the other team's reserves. Who you play with, and against, warps the raw number. Over one night it can be almost meaningless, which is why analysts rarely trust raw plus-minus on its own and reach for versions that control for context.
Net rating: points per 100 possessions
Net rating takes the same idea and standardises it: a team's offensive rating (points scored per 100 possessions) minus its defensive rating (points allowed per 100). Putting everything per 100 possessions removes the distortion of fast or slow games, so a +6 net rating means a team is six points better than its opponents over a normalised amount of play. Team net rating over a season is one of the most reliable summaries of quality.
On/off and adjusted versions
To judge a single player, analysts compare the team's net rating with him on the floor versus off it — the on/off split — to see how much better the team is with him out there. More advanced still are adjusted plus-minus and RAPM, which use whole seasons of lineup data to statistically strip out teammates and opponents. These are attempts to answer the hardest question in the sport: how much is one player actually worth?
How to read them
Trust these numbers in bulk, not in isolation. A season of net rating and on/off tells you far more than one game's plus-minus, and even then it is a clue rather than a verdict — pair it with what you see and with the box score. Used well, net rating is exactly the kind of possession-based, per-100 measure that underpins data-driven basketball analysis, including the fixture previews on SportMetrics.