I believe the plus-minus category is the least understood of hockey's commonly used statistics. It is basically used to determine whether more goals are scored by your team or the opposition when a particular player is on the ice. Tallies are kept for each game and each player's cumulative score is available on a daily basis, just like his scoring statistics.
Some goals scored do not count in a player's plus-minus rating. If your team scores a power play goal while you’re on the ice, you do not get a plus. If you are on the ice killing a penalty and the opposition scores, you do not get a minus.
Certain aspects of this statistic are clearly understood. When Bobby Orr compiled a rating of plus-124 for the 1970-71 season, it was a mark for the ages. When Orr and Wayne Gretzky posted substantially higher plus-minus ratings than any other player over long periods of time, it was certainly more evidence of their brilliance.
But there are a number of areas where caution must be used in using this statistic as a means of comparing hockey players. In particular, here are my areas of concern:
1. Even in the cases of Orr and Gretzky, the statistic often does not do justice to elite level offensive players. Such players spend a lot of their ice time on the power play, especially in the present day NHL. They often accumulate many power play points for which this statistic does not reward them.
2. A good ranking in this statistic often leads pundits to the conclusion that the player involved must be particularly good defensively. Let's use an example. Assume there are two players playing the same position. One of them is plus-10 after 40 games and the other is minus-10. Inevitably, pundits proclaim the player with the plus rating superior defensively. A closer examination often finds that the players were on the ice for an equal number of goals scored against their teams. The difference in the rankings is due to the fact the plus player was on the ice for a substantially higher number of goals scored by his team. In effect, it was his superior offensive performance that earned him a better plus-minus rating.
3. Every NHL team uses its best defensive players to kill penalties. If the top three defensive players from Team A are having a poor season, they are likely surrendering far more power play goals than the top defensive players from Team B, who are having a good season. Yet nowhere do plus-minus statistics account for this difference.
4. A player on any NHL team who consistently plays against the opposition's top line is likely to have a poorer plus-minus rating than a number of his teammates. Especially during home games, teams often try to match a "checking line" against the opposition's top line. Members of the checking line may grimace when they see their plus-minus figures. The numbers often do not reflect the value of such players to their teams.
5. A regular player on a top team is likely to have a better plus-minus ranking than a regular player on a poor team. The figures often have no significance if you project how two players in these categories might fare if they played the same roles on equal teams.
In general, the plus-minus statistic is interesting to examine so long as you use caution. Always take into account the different roles players on a particular team may be asked to fill. Make sure you account for the differences between particular teams before comparing the plus-minus rankings of their players. Finally, if you are using the ranking to determine the value of a particular player to his team, be sure to consider his contributions to power play and penalty killing units, which will not be reflected in this statistic.
The plus-minus statistic has value, but the trick is to consider it in the right context and realize its limitations.
Tom Thompson worked as head scout for the Minnesota Wild from 1999-2001 and was promoted to assistant GM in 2002, a post he held until 2010. He has also worked as a scout for the Calgary Flames, where he earned a Stanley Cup ring in 1989. He will be blogging for THN.com this season.