“The most important thing […] is to keep a clean sheet.”
Words along that line will sound familiar to anyone who has ever watched football managers comment on their pre-match intentions. A simple Google search for the terms “Eredivisie and “de nul houden” [Dutch equivalent of “keeping a clean sheet”] comes up with quotes from nearly all managers and players from all teams. Of course, keeping a clean sheet contributes highly to winning matches, but on the other hand, you’ll never win without scoring a goal. So how does this conflict of interest between defensive and offensive intentions translate in terms of results? How important is it exactly to prevent the opponent from scoring? And does it matter whether you’re playing a home or an away match? And what if the mission failed, how important is it to keep the opposing team from scoring a second goal, or when you’ve just scored yourself, to chase a second goal?
It’s questions like these that the following post will address. And it’s not the first to do so. It all started with a remark by Chelsea’s director of performance analysis, Mike Forde, when interviewed by Simon Kuper. Forde stated in the interview that “there is a stronger correlation between clean sheets and where you finish than goals scored and where you finish [in the Premier League].” This led Chris Anderson to quantify the relation between goals conceded and points won in an excellent post on Soccer by the Numbers.
As it turned out, Chris showed that a clean sheet was worth around three goals in the 2010/11 EPL. His excellent post was followed-up by Omar Chaudhuri, who expanded the data across two seasons in the four big leagues, to come up with around the same number.
Data from the first half of the 2011/12 Eredivisie is shown in the graphs below, which shows how many points team won on average in relation to the number of goals scored (upper graph) or conceded (lower graph).
Teams that failed to score a goal won on average only 0.14 points, while a singly goal scored resulted in 0.98 points on average. Teams that kept a clean sheet won on average 2.72 points. As the graph below illustrates, conceding a single goal lowered this number to 1.72, and another goal lowered this further to 0.72. This means that the incremental value of scoring a single goal over failing to score is 0.98 – 0.14 = 0.84 points, while the incremental value of conceding a single goal over keeping a clean sheet is 1.72 – 2.72 = 1.00. So in general, conceding a goal while the score is still 0-0 hurts the prospected points won from the particular match more than scoring a goal helps that same prospected goals tally.
Home and away matches
The following graph presents the same data, with the value of goals scored split out between home and away matches. This shows that single goals scored at home are worth more than single goals scored away from home (incremental value 1.00 vs 0.67), while second goals scored, reversely, are worth slightly more away from home (0.93 vs 0.82).
The final graphs depicts the points won according to the number of goals conceded, again split out between home and away matches. This shows that a clean sheet is worth slightly more at home than it is away from home (2.79 vs 2.64). To concede a single goal reduces the points won in home matches by 0.83, while in away matches this is much more costly at 1.15.
Applying this information
This raw data should provide invaluable input to the game plan going into a match. After all, one of the most important aspects of a game plan going into a football match is how to balance between offense and defense.
Data from this Eredivisie season show that the incremental value of a goal scored at home is 1.00, while the incremental value of a goal conceded at home is 0.83. Away from home, these figure are quite different. The incremental value of a goal scored away from home is only 0.67, while for goals conceded away from home this is 1.15.
In other words, a goal scored at home wins 24% more points than conceding a goal would cost the team. Away from home this is quite different, as goals conceded away from home cost 72% more points than a goal score would win. This stresses the general importance to attack at home while the scores is still 0-0, and defend the clean sheet in away matches in the same situation.
Defending a lead, or chasing more goals?
Curiously, things are a bit different when a goal has been scored. The incremental value between a single goal scored at home and a second one is 0.82, more or less the same as the incremental value of conceding a single goal at home, which was 0.83. Away from home, the incremental value between a single goal scored and a second one is 0.93, which is much higher than the 0.67 points added by scoring the single goal. So, when going a goal up away from home, there generally is more value to be gained by chasing the second goal than there was chasing that first one. Reversely, when playing at home, there is less value to be added in chasing a second goal than there was in chasing that first one.
A survey of 2011/12 Eredivisie data do more or less reproduce the numbers presented by Chris Anderson at ‘Soccer by the Numbers’ and Omar Chaudhuri at ‘5addedminutes’. Taking this analysis a bit further, the reduction in points won from a match when conceding a single goal is shown to be higher away from home.
Looking at incremental values between 0, 1 and 2 goals scored should help managers to make a rational choice whether to focus on offense or defense going into the match, but even more so when going a goal up or down. It seems, in away matches in the Eredivisie, defending a 0-0 score line generally seems a wise thing to do, but when going a goal up it seems wise to chase the second goal a bit more than you did the first one. The reverse is true for home matches, as a single goal scored at 0-0 wins much more points than a single goal conceded would cost, while a second goal scored wins about as much as a single goal conceded would cost.