Goal Difference
For a player one can work out the nett goals scored by his side whilst he is on the field. Targeting these players for acquisition can lead to a greater difference in goals scored vs goals given away for the team.
Current Consideration to Player Goal Difference by Managers in the League
Managers base a considerable amount of a player's wage level on the player's performance and how well he helps the team win. This "performance" should therefore be strongly correlated to player goal difference.
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The level of wage is a close approximation on how highly managers value a player's strength. Managers have very good knowledge on how strong a player is, therefore if we use player wages as an indicator of players strength, we can compare goal difference to wages to see how closely related player strength is to player goal difference.
We worked out the goal difference of players* and to test how well this lines up to how valuable managers perceive them, we plotted the goal differences (horisontal axis on the chart) to the weekly salaries (vertical axis on the chart). For the highest salaried players, one can see that their goal difference are also highest. - This may indicate that managers do extreme due diligence on whether their most expensive players are truly worth the expense, but not so much for the lower-salaried players, leading to a disconnect between goal difference and perceived value/salary.
Correcting for Player Fan Popularity
For commercial reasons, the best players may have their salaries inflated for being fan favourites too, so moving them away from their pure worth in goal difference. To correct for that one replaces the value figures with their rank amongst the other players' salary. This makes the relationship in the chart more straight and therefore direct.
Statistical Significance
(The statistical correlation** between the goal difference and the salary has been worked out and it is quite strong. A random set of numbers only have two in a quadrillion chance of having such a strong correlation. (So you can be sure goal difference correlates with how valuable managers view players))
Minimum Playing Time Needed
We statistically determined the total time a player needed to have stayed on the pitch to get his most accurate goal difference
Correcting for Opponent Strength
In calculating goal difference, we additionally adjust the goal difference for the player, based on the strength of the opposing players he played matches against. This adjustment makes players fairly comparable to each other, even if they played against different opponents (for example if the opposing team misses their star defender for one of the matches, or the players being compared play against totally different clubs altogether).
*German Bundesliga 2018/19 season
**Spearman's Rho rank correlation
Samples
Displayed here is the straight-forward goal difference and its rank amongst all Bundesliga players.
This goal difference is then adjusted by the strength of the opponents the player played against and a new ranking is achieved based on the adjustment.
The arrow indicate whether the adjustment of the goal difference made the player improve his ranking (green), or worsen (red)
(the adjusted goal difference has a higher correlation with the salary that clubs pay)
League Samples
Simple goal difference results
This is a sample, so only every 7'th player on the ranked list is shown.
(For matches from start of 2021 season till now.)
