However,this approach has problems.Defences will face every other attack in the league twice over the season,but they are stuck with their own forward and midfielders for each game.And the quality of a team's other outfield players can impact on the amount of defending a team must do and this in turn can be reflected in the amount of goals they concede.
To take an obvious example.Manchester United defenders at Old Trafford are unlikely to find themselves under the same amount of pressure experienced by their visiting counter parts.United's forward and attacking midfielders will likely have possession of the ball for a longtime,during which there will be little or no defending for United to do.In short overall team quality makes defending easier and not all teams can afford or attract the kind of quality throughout the team enjoyed by Manchester United and the other top teams.
We therefore need to use numbers that initially at least incorporate a team dimension before we try to decide how effective certain parts of a team perform their various tasks.
Average goal difference (AGD) per game is such a stat.It's the difference between the average number of goals per game a team scores and the average number of goals they concede and it is very strongly correlated to success indicators such as final league position.Over the 2010/11 season,Man Utd's average goal difference per game was +1.08,compared to -0.71 for relegated West Ham United.
AGD incorporates both average goals scored and allowed,so it is a genuine team statistic.A team can accumulate positive AGD by scoring goals and although their goal difference will always be eroded by conceding goals it is still advantageous to allow as few goals as possible to be scored against your side.
I've taken each EPL team's seasonal AGD and plotted the numbers against the average number of goals per game allowed by those teams since the 1999/2000 season.As you can see from the graphic below AGD is strongly correlated to goals conceded.We would of course expect it to be.
From the graph we can derive an equation that relates the amount of goals allowed by a team for a certain level of average goal difference.This relationship gives us an opportunity to see how much of a team's AGD is derived from it's defensive abilities compared to the typical expectation for the EPL.Teams that are overachieving in relation to the norm can be considered to be performing extremely well,defensively regardless of their overall level of talent.
