Effect Size Index (ESI) tracks how well a team performs when a player is on the park, as opposed to their scoring/concession rates when he is not.
A player who plays every minute of every contest for his club - usually a goalkeeper or centre-back, will have an ESI of 0.000, as this statistic charts the difference between the team's performance when a player is on the pitch (in terms of frequency of goals scored/conceded) and compares it to his club's overall rates of goals scored/conceded. Thus, a player who plays every minute of the season - or who's Individual Plus/Minus per 90 minute matches that of his club - has an ESI of 0.
Take, for example, Jack Wilshere (please). Arsenal score less frequently when he is on the pitch, while conceding more often: Arsenal have GD this season of +7, but while Wilshere has been on the pitch the Gunners have been outscored by a total of 10 goals to 4 (making Wilshere's +/- a poor -6). While this might be bad luck, bad teammates or bad Jack, and keeping in mind that 0.000 is for want of a better word "baseline", his ESI is a phenomenally bad -1.527.
You can find the ESI of every player who has managed 450 minutes on the pitch this season by clicking this link. Player are grouped by position, and arranged in order of best to worst ESI.