I recently reread Simon Kuper's column about statistics in soccer, in which he quotes Mike Forde, Chelsea's Performance Director, who has worked hard to find ways of making statistical analysis useful for his club. It's an interesting piece, but what struck me as particularly interesting for soccer analysts was this quote:
"The holy grail would be discovering the key to victory. “I do not think we are there yet,” Forde admits. But he says: “If you look at 10 years in the Premier League, there is a stronger correlation between clean sheets and where you finish than goals scored and where you finish.”"
Of course, I was curious to see if there is something to clean sheets that's worth pursuing, as Forde claims. So here we go.
One way to think about it is to see if we can quantify the point value of a clean sheet. It may help to think about it this way: a clean sheet guarantees a team at least one point from a match and potentially gives it three (in case the team scores a goal). So I cranked up the old EPL dataset and calculated the average amount of points associated with a clean sheet (and goals allowed per match generally). Here's what we see for the 2009-10 EPL season.
Forde is right on the money (kudos to his number crunchers). Clean sheets (denoted by the 0 line, which indicates no goals allowed in a match) on average produce almost 2.5 points per match. And even only 1 goal allowed still gives a team slightly more than 1.5 points on average. But by the time we get to 2 goals allowed, point value rapidly declines. And once the other team scores 3 or 4 on a team, the point value rapidly goes to 0.
Compare this with offensive production (see below). You might think that scoring at least one goal will help you as much as not letting one in. You'd be wrong. Scoring 1 goal only gives you about 1 point from a match, on average. Compare that to 1.5 points for allowing one goal only. So the point value of 1 goal allowed is 50% greater than the point value of 1 goal scored, according to this calculation.
Another way to think about this is to ask how many goals a team needs to score to produce the points produced by a clean sheet. The answer for the 2009-10 EPL season is slightly greater than 3. So a clean sheet produces about as many points for a team as scoring 3 goals on offense.
"The holy grail would be discovering the key to victory. “I do not think we are there yet,” Forde admits. But he says: “If you look at 10 years in the Premier League, there is a stronger correlation between clean sheets and where you finish than goals scored and where you finish.”"
Of course, I was curious to see if there is something to clean sheets that's worth pursuing, as Forde claims. So here we go.
One way to think about it is to see if we can quantify the point value of a clean sheet. It may help to think about it this way: a clean sheet guarantees a team at least one point from a match and potentially gives it three (in case the team scores a goal). So I cranked up the old EPL dataset and calculated the average amount of points associated with a clean sheet (and goals allowed per match generally). Here's what we see for the 2009-10 EPL season.
Forde is right on the money (kudos to his number crunchers). Clean sheets (denoted by the 0 line, which indicates no goals allowed in a match) on average produce almost 2.5 points per match. And even only 1 goal allowed still gives a team slightly more than 1.5 points on average. But by the time we get to 2 goals allowed, point value rapidly declines. And once the other team scores 3 or 4 on a team, the point value rapidly goes to 0.
Compare this with offensive production (see below). You might think that scoring at least one goal will help you as much as not letting one in. You'd be wrong. Scoring 1 goal only gives you about 1 point from a match, on average. Compare that to 1.5 points for allowing one goal only. So the point value of 1 goal allowed is 50% greater than the point value of 1 goal scored, according to this calculation.
Another way to think about this is to ask how many goals a team needs to score to produce the points produced by a clean sheet. The answer for the 2009-10 EPL season is slightly greater than 3. So a clean sheet produces about as many points for a team as scoring 3 goals on offense.
If I'm not completely off the mark (and I could be - wouldn't be the first time), then this raises an interesting question: why are strikers valued significantly more highly than defenders and goalies? Is that about offense being the best defense, or is there more going on? Either way, Mike Forde knows what he's talking about.
