The Plus Minus statistic is one which can be applied to a variety of team sports, and within the context of Football is essentially an individual goal difference, calculated purely on the time the player spends on the pitch. So, if Team A are leading Team B 2-0 and then replace Player C with Player D, then Player C will score +2 for that match. Should that match end 2-1, then Player D will score -1, with any player from Team A playing the full match scoring +1, and any player from Team B playing the full match scoring -1.
Therefore the Plus Minus statistic can be used as a way to measure the individual value of a player, though as with any statistic requires context. One area of particular relevance is the amount of time a player spent on the pitch to achieve their particular rating. Particularly at the extremities of the table, the overall Plus Minus statistic will be skewed towards those with the most game time. In a championship winning side, it would be very difficult for someone only playing 10 games during the season to record a higher Plus Minus rating than a team mate playing 30+ games, regardless of how influential they may be whilst on the pitch. Therefore alongside the overall Plus Minus rating, this site also provides an average per 90 minutes played as a way to counteract this.
I first came across the Plus Minus statistic when watching Ice Hockey, where the statistic is only calculated when the teams are ‘evenhanded’, i.e. both teams have the same number of skaters on the ice. If a goal is scored whilst one team has a numerical advantage, that goal will not be counted towards the Plus Minus statistic.
However, for this blog, all goals scored will be included within the Plus Minus statistic. Football and Ice Hockey differ in several ways – a Football match is longer, is 11 per side rather than 6 per side, does not feature a Penalty Box or Sin Bin and typically results in fewer goals per match overall. Each of these factors meant that drawing a distinction for periods of numerical advantage does not provide the same level of statistical value in Football as it does in Ice Hockey, which is therefore why I have chosen not to compensate for it here.
Rotation and Inconsistency
If a player plays every minute of the season, their Plus Minus statistic will therefore be the same as the team’s overall goal difference. The less time a player spends on the pitch, the greater potential there is for variance from the team average, both in terms of the overall rating and per 90 minutes. There is also an even greater potential for variance if the team’s results are inconsistent, meaning that the greatest spread of ratings will often be found with mid-table teams who regularly rotate their starting line up.
Studying the Plus Minus statistic highlights the impact substitutions can have on the results of a team. Positive substitution impacts are defined as players recording a positive rating, having replaced a team mate who recorded either a negative rating or zero, whilst a negative substitution impact is defined as a player recording a negative rating having replaced a team mate who recorded either a positive rating or zero. In regards to what this actually means, a positive substitution impact will result in a team either converting a drawing or losing situation into a winning position, converting a losing situation into a draw or reducing the deficit of defeat. A negative substitution impact will result in a team either coverting a winning or drawing situation into a losing position, converting a lead into a draw or reducing the margin of victory.
Therefore over the course of the season, if the number of positive impact substitutions comfortably outnumbers the number of negative impact substitutions, that would indicate a team with a strong bench, a manager who understands how and when to use substitutions, or perhaps a combination of the two factors. However, it could equally indicate that the starting line-up either has a habit of starting games slowly, or out-perform opponents in the latter stages of matches due to superior fitness or dominance of possession.