One of the happy discoveries of working on this site was that through the process of calculating the Plus Minus statistic, other statistical trends become apparent. Nowhere is this more the case than in regards to substitutions. Substitutions are fundamental to the Plus Minus statistic because you need to know what the score was when a player went off, and how that scoreline changed by the final whistle for the player coming on. I also record the number of minutes played too in order to be able to calculate the Plus Minus per 90 minutes, though that is information which is also available elsewhere.
Through recording data in regards to substitutions, broader trends in related to substitutions also become apparent, such as positional trends. Whilst you will see examples of defenders being substituted when a team is chasing a goal, or brought on when a team is trying to defend a lead, tactical substitutions involving defenders are much rarer than those involving attacking players across the board. What is less universal is when a manager will choose to use their substitutions. Unai Emery for example was often prepared to make substitutions at half time or early in the second half, where as someone like Roy Hodgson generally waited until later in the game to make changes.
Substitutions can cause big swings in terms of a player’s Plus Minus ratings, particularly for players who rarely complete 90 minutes. To highlight just one example, Gerard Deulofeu can be considered as a regular for Watford last season, playing 30 times in the league. Yet he only completed the full match on 6 occasions, and there was a 12 goal swing between his rating of +5 and Ben Foster’s rating of -7 (Foster played every minute of the league season). Watford’s goal difference in the games Deulofeu didn’t play was actually +2, meaning that in the games he did play, there was a 14 goal swing between when he was on the pitch and when he wasn’t. Though not always the case, you’ll often find the players at the top and bottom of a team’s rankings are those that are often substitutes or substituted.
Therefore in order provide greater detail regarding substitutions for the 2019/20 season, substitution impact will be recorded and updated along with the Plus Minus ratings. Whilst the Plus Minus ratings will inform this statistic, it can be thought of separately too. Put simply, substitution impact reflects whether a team’s fortunes improved, worsened or stayed level following a substitution, and therefore measures how ‘successful’ a substitution was.
As with many measures of success, that success may be relative. Whilst a substitution may deliver an improvement, that improvement may be from a team trailling 4-0 when a player is introduced to losing 4-1 at the final whistle. The substitution represents an improvement, though only in terms of goal difference, which could prove decisive at the end of the season but often is not. However, there may also be occasions when that one goal improvement turns a defeat into a draw, or a draw into a win, and generally if a team has a trend of making more successful substitutions than unsuccessful substitutions, then that would be expected to translate into a net points gain over the course of the season.
Substitution impact is not the only new statistic which will be added to the site next season though, with more to be revealed as we approach the start of the season.