Plus-minus statistics exist for other fluid sports such as hockey and basketball. Now, they also exist here for Major League Soccer. Plus-minus simply measures the difference between a team's output and the opponent's output while a given player was playing. The metric is typically scaled per game or per unit time. In hockey, goals are used to measure the differential, and in basketball points are used.
Initially, we're rolling out basic plus-minus statistics using goal differentials and expected goal differentials, and later we will include adjusted versions taking the gamestate into account, as well as a player's teammates. Our plus-minus statistics are scaled per 96 minutes, the average length of an MLS game.