Wednesday, 28 September 2011

Defensive Performance and Transfer Window Purchases.

At first glance it seems relatively easy to decide which teams have he strongest defences in the EPL.Goals allowed per game is almost universally used to demonstrate defensive ability.A goal conceded is to a large extent down to a team's defenders,so it stands to reason that the less you concede,the better you are.To add even more weight and justification to this choice is that,unlike say the NFL each team plays all the others twice,therefore the schedule is almost identical for everyone.

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.

An example or two will hopefully make the method easier to visualise.

Man City's goal difference per game in 2010/11 was 0.71 and a typical team with that AGD would allow on average 0.97 goals per game.The Cup holders allowed just 0.87 goals per game or around 10% less.Therefore,they can be classed as an exceptionally good defensive team.

By comparison,Arsenal's 2009/10 team,with an AGD of  1.11 would have expected to allow 0.84 goals per game and not the 1.08 goals they actually did.That's not to say Arsenal aren't a good side,they clearly are.But it does indicate that their AGD is heavily reliant on their goal scoring and not their goal stopping.To further illustrate how this method pinpoints defences that are performing above expectations compared to an average team of similar quality,here's the top five and bottom five from last season's EPL.

Five Over and Under Defensive Performers from 2010/11.

Team. Goal
Difference per Game.
Goals Conceded per Game.
Goals Conceded per Game.
MAN CITY. 0.71 0.87 0.97
FULHAM. 0.16 1.13 1.20
BIRMINGHAM. -0.55 1.53 1.58
STOKE. -0.05 1.26 1.30
CHELSEA. 0.95 0.87 0.89
--------------- --------------- --------------- ---------------
MAN UTD. 1.08 0.97 0.84
NEWCASTLE. -0.03 1.50 1.29
ARSENAL. 0.76 1.13 0.95
WBA. -0.39 1.87 1.49
BLACKPOOL. -0.61 2.05 1.61

Buying in the Transfer Window to address your Weakness.

Man City's defensive performance outperformed that of a typical team who would finish the season with an average goal difference of 0.71 goals per game and that over performance meant that it was the attack that let down their title aspirations.If their attack had performed to similar levels as their defence their goal difference would have risen to around 0.8 goals per game.Solid Champions League qualifying form,but still short of challenging for the title.Therefore,investment in both defence (Clichy)and massive investment in forward players (Nasri and Aguero) was both predictable and sensible.Both Stoke and Fulham splashed the lions share of the money they spent during the transfer window on strikers.Buying Crouch and Jerome and Ruiz,respectively.Improvement in attack,coupled with a continued strong showing at the back should see both teams as established mid table and upwards outfits.Chelsea also seemed happy enough with their defence and bought in Mata and Meireles.

At the defensive under achievers end of this mini table,Manchester United spent twice as much on defenders as they did on attackers.Arsenal replaced their two departing attacking stars,but acknowledged their defensive frailties by investing in Werder Bremen's Mertesacker.Newcastle appeared reluctant to spend much money in the window,Blackpool's transfer dealings were dictated by their relegation and only WBA seemed to ignore their implied defensive weakness by buying striker,Sean Long from Reading.In mitigation the Baggie's manager,Roy Hodgson has shown himself capable of producing outstanding defence,most notably at Fulham in 2009.

If we now use the same methodology for each season the current members of the EPL have spent in the top tier since 1999 and average the results,we can see which teams have relied on their defence to allow them to achieve their finishing positions and which teams have seen their attack take the strain.

Team. Average %
of  Goals "saved" by Defence compared to Team Standard.
Best Season for Defensive Contribution. Worst Season for Defensive Contribution.
CHELSEA. 8.9 2004/05 2009/10
LIVERPOOL. 8.7 2005/06 2000/01
STOKE. 7.4 2009/10 2010/11
SUNDERLAND. 4.6 2001/02 1999/00
FULHAM. 4.2 2008/09 2004/05
WIGAN. 3.2 2008/09 2009/10
ASTONVILLA. 2.2 2006/07 2007/08
EVERTON. 1.2 2007/08 2009/10
MANCITY. -0.1 2009/10 2010/11
BOLTON. -0.7 2005/06 2010/11
BLACKBURN. -0.9 2004/05 2003/04
WBA. -1.1 2005/06 2010/11
WOLVES. -3.2 2009/10 2010/11
MANUTD. -3.8 2004/05 1999/00
NEWCASTLE. -5.9 2006/07 2001/02
TOTTENHAM. -6.3 2004/05 2007/08
ARSENAL. -9.7 2004/05 2003/04

Chelsea's defensive strength came to the fore during the Mourinho years and during each of his three seasons the defence contributed a larger proportion of their goal difference than would be normal for a team consistently challenging for the title.By 2009/10 they had flipped to become an attack orientated side.
Liverpool only had three seasons out of 12 when the attack contributed more than the defence,while Stoke have seen a bigger defensive contribution in each of their three completed EPL ventures.They were especially strong in their anticipated difficult second season.

Man Utd struggled defensively in the early seasons of the decade and latterly during the 2010/11 season,whilst Arsenal's insistence on favouring attack over defence is well illustrated.Eight of their 12 EPL seasons were dominated by a stronger than expected attacking contribution for a team of Arsenal's goal difference.

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