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October 21, 2013

Goalkeeper Save Ratings EPL 2012/13

By Colin Trainor

During the Summer I cast my first analytical eye at Goalkeepers in a piece over at my old Statsbettor site, this article can be found here.  In that piece I analysed the saves made by Premier League goalkeepers during the 2012/13 season and I concluded that Julio Cesar from QPR was the best shot stopping keeper last season.

Just about the only other person within the analytics community that cares about the poor old goalkeeper is Paul Riley, and he produced this excellent piece yesterday.

As tends to be the difference in our styles, I am tackling the same subject in a slightly more quantitative way than Paul did yesterday, but I’ve no doubt that we’ll end up at almost the same place.

All regular readers of my writing will know that Constantinos Chappas and I have developed a number of models that numerically  estimate the probability of a goal being scored.  We use Opta data served up by Squawka and / or StatsZone in order to estimate these probabilities.  As a result we do not have any data on defensive or goalkeeper positioning, but our models take into account whatever “on the ball” information is significant in determining the probability of a goal being scored.

ExpG2

In an article published last month I introduced a new metric that Constantinos and I calculated, ExpG2.  ExpG2 tells us the probability of a shot being scored AFTER the shot has been struck.
In short, ExpG2 is arrived at by looking at where and how the shot was taken and where it was eventually aimed for.  The further away from the centre of the goal the shot is placed the higher the ExpG2 value.

A blocked shot or a shot off target will always have an ExpG2 of zero because once these shots have been struck there is a 0% probability of them resulting directly in a goal.  A close range shot that is aimed towards the corners will have an ExpG2 value approaching  1.00.

According to my records there were 10,562 shots in last season’s EPL and 97 of them had an ExpG2 value of >0.90 (ie they had a greater than 90% chance of being scored).  89 of them were actually converted which suggests that the ExpG2 values are pretty well calibrated.

Rating Goalkeeper’s Saving Performances

After we created the ExpG2 values, their potential to rate the saves (or lack thereof) by goalkeepers became obvious to me.  In simple terms, we now have a numerical value of how difficult each save was to make.
At this point I must acknowledge once more that these values do not take into account defensive pressure.  However, the work that has gone into them and the detail included in their calculation means that I am not aware of any other goalkeeper save rating system that is more detailed.

The table below includes all Premier League goalkeepers that faced more than 100 shots last season.  It’s not important to know how many of the +100 shots were on target as we compare actual goals conceded to the Expected number conceded.

ExpG2 is the amount of goals that our model expected the goalkeepers to concede based on the type of shot and shot placement
Goals are the number of goals conceded by the respective GKs
Save Efficiency is (ExpG2  / Goals), with a higher number signifying less goals than expected were conceded

The first thing to note is that the 7 keepers at the top of the list are the same 7 that appeared atop my ranking table in my previous look at this topic, even though my method of evaluation was slightly different.  So that’s a good start.

The more exact data that has been used in this exercise sees David De Gea achieve the best shot stopping performances last season.  The shots he faced suggested that he should have conceded 8.6 goals more than he actually did; this along with United’s ruthless attacking efficiency last season would have been major reasons that help explain how Fergie could succeed where David Moyes is so far failing.

You’ve got to feel for Gerhard Tremmel, I’ve him in second place in terms of saves in the Premier League last season, yet he can’t even get a game.

Mignolet, Julio Cesar and Begovic where the only other goalkeepers to have a Save Efficiency of more than 125%.  I haven’t yet ran the current season’s saves through ExpG2 (I intend to do this in the coming days) but it certainly feels that Mignolet has brought his super saving performances to Liverpool this season.

At the other end of the table you can see that Wigan struggled with whichever keeper they picked (as did Southampton).  Looking at the values returned by the two Man United keepers it is difficult to think that at one stage last season Lindegaard appeared to be the first choice custodian at Old Trafford.  Unfortunately for him, his and De Gea’s season went in entirely different directions.

In North London it is clear to see why Spurs decided to bring in Hugo Lloris to replace the aging (and ailing) Brad Friedel.  Although at less than 100% Save Eff the French man didn’t return sparkling numbers himself.

It strikes me as unusual to see the England GK (Joe Hart) having a worse than average record of saving shots last season, his Save Eff was just 95%.

Shots From Inside the Penalty Area

To give us a little more detail we can divide the shots faced between those coming from inside and from outside the penalty area.  This time we’ll concentrate on shots that were taken from inside the Penalty Area.  Again, the table below includes all goalkeepers that faced more than 100 shots from inside the area.

For shots faced from inside the penalty area De Gea appears in just 4th position.  Three keepers (Begovic, Cesar and Mignolet) returned almost identical performances at the top of the rankings with Save Eff values of 127%.

Joe Hart noticeably climbs the rankings on this measure as he conceded 2.5 less goals from close in shots than would have been expected.  Hart’s strength appears to be stopping the closer in shots (perhaps due to good reflexes).
Reina underperformed badly when faced with shots from inside the area.  The information available to me suggests that he allowed 2 more goals that he should have done.  In both of these tables it can be seen that, from a shot stopping point of view, the replacement of Reina with Mignolet was a clear upgrade by Liverpool.

Shots From Outside the Penalty Area

The table below shows all goalkeepers that faced more than 50 shots from outside the penalty area last season.

Due to the volatility implicit with long range shooting there is a much greater spread of performances than we seen with the inside penalty area saves.  As a result of this we perhaps need to be careful with any conclusions that we draw from this dataset as a keeper allowing or stopping just 1 additional goal could have quite an influence on the values in this table.

Alex McCarthy of Reading did really well when dealing with long range shots as he only conceded 1 goal instead of the 4 that the shots aimed at him would have suggested.
Tremmel, Lloris and De Gea all conceded at rates of half (or less) that would have been expected.

But let’s cast our eyes right to the foot of this table.  Joe Hart is in penultimate place in terms Premier League goalkeepers saving long range shots.
Last season he conceded 9 goals from these shots, when our model would have said that he should have allowed 5 of those efforts to result in a score.  This finding agrees with what Paul Riley concluded in his piece from yesterday, Joe Hart had a problem last season with stopping long range efforts.  With him being above average in terms of stopping closer shots perhaps his footwork or anticipation lets him done on these long range efforts.

To put Hart’s (lack of) performance in context, Alex McCarthy conceded 1 goal and Joe Hart 9 last season from shots outside the area when all quantitative data tells us that Hart should have just conceded one more goal.  That’s quite the difference……..

Repeatability

Like all the analytical work I perform I ask myself whether I’m reporting on something that just happened or whether these values are repeatable and thus they can be used in terms of picking teams or lining up transfer targets.
As we are just in the infancy of soccer analytics I can’t answer that question right now.  But with time, and the ability to run this type of analysis over previous seasons I hope to be in a position to determine whether there is any repeatable material difference in the ability of Premier league goalkeepers to save the efforts headed their way.

Article by Colin Trainor