Stats & Links: Boateng + Thiago = success?
The plus-minus rating was introduced on Miasanrot as early as a few years ago. This rating is already wide-spread above all in US sports like basketball or ice hockey, and is often used for evaluating players. In short, it describes the goal difference of a team during all minutes in which the player was on the pitch.
Subsequently the plus-minus rating can be applied against the average rating of the team in order to come to the actual plus-minus, i.e. a margin that corresponds to the influence of the player on the team.
An initial step requires calculating the goal difference when a certain player was on the pitch. This value will be henceforth known as plus-minus.
In the graph, the goal difference for all players in the 2017/18 season just gone is represented. Here the ratings are additionally plotted along the number of minutes played. A red line shows the team average, which was at 1.89 goals for the season. So Bayern scored precisely those 1.89 goals more per game on average than the opponent.
The different coloured areas correspond to the 90%, 95%, and 99% percentiles and can be understood as statistical quality criteria. Practically that means that the likelihood that Boateng only has such a good rating coincidentally is lower than ten percent, because his rating is within the 90% percentile.
The player-specific rating then still has to be offset against the team-average to get the actual influence. This value is designated henceforth as delta-plus-minus. In the table below, these ratings are now represented:
For a better overview, positive delta-plus-minus ratings are represented in green and negative ones in red. Here the data for the 2017/18 were evaluated. Like all statistics, the pure numbers still require an interpretation and classification.
If when evaluating you leave out the anomalies with few minutes like youngster Mai, returning Starke or long-injured Neuer, the ones who stand out are Alaba and Boateng. The two long-time key figures have with +0.63 and 0.48+ respectively absolute top ratings in the team. With the two on the pitch, Bayern score an average of half a goal more than without them.
With Boateng the precise analyse shows that he was only on the pitch for one defeat in the whole 2017/18 season – in the Champions League semi-final first leg against Real Madrid. And even in that game the German international left the pitch with an injury when it stood at 1-0 to Bayern.
Boateng showed as early as last season that he couldn’t reach his absolute top level anymore due to his injuries that constantly came back. But the Berlin-born defender seems to be an important anchor in Bayern’s defence.
A similar statement can be made for Alaba. The Austrian was also missing in a lot of games due to a range of injuries. His absences were insufficiently compensated for. In the eleven games that the former Bayern youth player missed, the side fell to three of their four defeats of the season.
In two of the three defeats, Alaba was covered for by Rafinha, which conversely explains the low rating of -0.33 for the Brazilian. The actual replacement Bernat comes in with a good rating with +0.23.
With the midfielders, most of the players exhibit a negative plus-minus-delta. In terms of the positives, merely Thiago with +0.21 and Müller with +0.22 stick out. As a result, the Bayern veteran outstrips his direct “competitor” James by 0.45 net goals/game.
It’s particularly with James and Müller that further analysis would be interesting, one that demonstrates how the plus-minus changes when only one of the two are on the pitch or indeed both of them together.
While Thiago’s influence on Bayern’s game is sufficiently appreciated here on the blog as well, in light of his plus-minus the limits of how much can be read into the rating should be explained.
The Spaniard, with +0.21, has a strong plus-minus rating, especially in comparison with his team-mates in midfield like Martinez (-0.16) or Tolisso (-0.36). Here too an organised view of the games completed by the number six helps. In total Thiago missed 15 Bundesliga games. Of these, two were lost. The points average with and without the midfield director is exactly the same at 2.47 points/game.
So where does the good plus-minus rating come from for Thiago, who is better than his team-mates in comparison? With the Spaniard in the line-up, Bayern won six times by more than three goals, doing so not once without him.
The plus-minus rating, then, is suited to comparing various players and evaluating their performances. However, it should not be used without considering further statistics like points achieved per game, because otherwise individual conclusions can be weighted too heavily.
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