The Definition and Purpose of the Plus/Minus Statistic in Hockey

In the National Hockey League (NHL), each player has a plus/minus statistic that is used to measure his skill as a defensive player relative to other players. This statistic can also be referred to as a plus/minus ranking. The symbols +/− or ± also refer to the plus/minus statistic.

How Is it Calculated?

When an even-strength or shorthanded goal is scored, every player on the ice for the team scoring the goal is credited with a "plus." Every player on the ice for the team scored against gets a "minus." The difference in these numbers by the end of the game makes up each individual player's plus/minus ranking. A high plus total is taken to mean that a guy is a good defensive player. 

To clarify, an even-strength goal means a goal that is scored when there are the same number of players on each team. A shorthanded goal is a goal scored by the team that has fewer players on the ice than the opposing team due to penalties. 

In calculating the plus/minus statistic, power play goals, penalty shot goals and empty net goals are not taken into account. Power play goals are scored by the team that has more players on the ice than the opposing team due to penalties. A penalty shot, which occurs when a team loses a clear scoring opportunity because of a foul, is a chance for a player to score a goal on the offending team without any opposition except the goaltender. Empty net goals are when a team scores a goal when there is no goaltender present at the net. 

Origins

The plus/minus statistic was first used in the 1950s by the Montreal Canadiens. This NHL team used this ranking system for evaluating its own players. By the 1960s, other teams were also using this system. During the 1967-68 season, the NHL officially started using the plus/minus statistic. 

Criticism

Because the plus/minus statistic is a very broad measurement, there has always been disagreement over how useful it is. The plus/minus system is criticized for having too many moving parts and variables. Meaning, the ranking is determined by many factors out of the control of the player being evaluated.

More specifically, the statistic depends on the team's overall shooting percentage, the goaltender's average save percentage, the performance of the opposing team and the amount of time an individual player is allowed on the ice. Because of the way the plus/minus statistic is calculated, a player with the exact same skillset can get drastically different plus/minus rankings. 

Thus, many hockey players, coaches and NHL commentators have complained that the plus/minus statistic is not useful when it comes to comparing individual players or evaluating a player's skill.