Saturday, 11 February 2012

The EPL's Best Keeper.

*A season ending version of these number can be found here

The most exposed playing position on the football pitch is that of goalkeeper.One minute you're flying through the air at Wembley to turn a goalbound Balotelli effort past the post with your "wrong hand" and the next you're allowing a speculative thirty yarder to squirm over the line at a windswept Britannia Stadium.A specialist in a team of comparative all rounders,any mistake or act of brilliance made by the last line of defence is often vital to the outcome of the game,rarely forgotten, and one of the first statistics to be paraded at game's end.Just as strikers tend to live or die by their goals scoring records,keepers are judged almost entirely on the saves to goals ratio.A keeper is the footballing equivalent of an NFL kicker,an undervalued specialist asset who is responsible for half the scoring and isn't allowed to let his position defining statistics drop too much before termination of contract becomes a distinct possibility.

Thomas Sorensen prepares to be fooled by randomness.
So how fair is it to judge a keeper on the only set of statistics he can be expected to produce.As with most tentative steps into player evaluation  we must accept and acknowledge that the data we have is inevitably going to be of limited sample size and therefore contain a component that is dependent upon the the player's actual talent and a second contribution that is going to be down to random chance.The smaller the sample size,the larger the contribution from randomness.To illustrate the point consider an untested keeper who plays the last ten minutes of a game because of injury and during that time he faces one reasonably difficult shot,it's around a 70% chance that he will make the save,(70% is around the average save rate for EPL keepers over recent seasons).As he stretches to make the stop,one of two things can happen.He can make the save and for the time being become the keeper with the highest save to shot ratio in the league or he can concede the goal a fall to rock bottom.

If we are using save to shot ratios to rank keepers then neither evaluation of our one shot wonder is likely to be accurate.One is overly generous,the second unfairly dismissive.Lack of opportunity is the main culprit for our indecision and the usual solution is to exclude players who have failed to reach a minimum number of shots faced.However,even a seemingly large number of shots faced are going to contain varying degrees of random chance,so rather than discriminate against those small sample sized wannabees,lets try to use every piece of goalkeeping data at our disposal.

If our hero makes the save,it shows a level of competence and a level of luck.He's lucky he didn't slip as he prepared to dive,that he wasn't unsighted,the ball wasn't wickedly deflected or the shot wasn't so good it was unsaveable.If we had no prior knowledge of his ability before this save,the best guess we could make would have been that he was most likely to be an average EPL stopper.Add in the single save and we can elevate him slightly above average.As our keeper faces more shots on target his opportunity to impress or depress increases and we can start to develop and opinion that becomes increasingly defined by his skill rather than his good or bad fortune.

The save to shot ratio of individual EPL keepers rarely drops below 60% or breaches 80% and more commonly is around 70%.We can use a keepers successful shot saving efforts and his unsuccessful goal allowing failures to determine how likely he is to be a member of each particular shot saving group of keepers.Is he really a 60% stopper whose present 70% saving rate owes a lot to randomness and limited sample size or is he an 80% who's current statistics are being underwhelmed by bad luck.The less information we have,the more we have to drag those raw percentages back towards the average of all keepers in the group.In short our 100% success or failure,one shot guy gets a lot of benefit of the doubt if he allows the goal and a healthy dose of "wait and see" scepticism if he pulls off the stop.And what goes for him also applies in steadily declining degrees to his more tested colleagues.

Below I've taken virtually every keeper who has pulled on a glove in anger this season and corrected his raw save to shots ratio to account for the amount of action he has seen.The less shots faced the more his numbers have been dragged either upwards or downwards towards the average for all keepers over the last couple of completed seasons.I've also added a final column to show which keepers have been subjected to the biggest correction by virtue of their varying shot stopping sample sizes compared to their raw numbers.

Chelsea's Hilario is near to the top of most conventional lists because he's saved 80% of shots on target.However,he has only faced 10 such efforts,but rather than exclude him from the list we have taken those 10 shots and credited him with a spot just above the league's average.Only time and opportunity will move him and improve our estimation of his true talent.For now he's above Petr Cech,but not as overwhelmingly so as he was if we had looked solely at each keeper's current save to shot ratio.

Shot Adjusted Save Percentage for 2011/12 to the end of January.

Keeper. Team. Shots Faced. Save
Save %.
Hart. Man City. 80 77.5 74.6 +2
Mignolet. Sunderland. 57 77.2 73.6 +2
Friedel. Spurs. 101 76.2 73.5 +2
de Gea. ManU. 73 76.7 73.4 0
Lindergaard. ManU. 22 81.8 73.4 -2
Hennessey. Wolves. 151 73.5 72.4 +5
Stockdale. Fulham. 53 75.4 72.3 0
Vorm. Swansea. 107 74.8 72.0 +1
Schwartzer. Fulham. 71 74.7 71.9 +1
Cerny. QPR. 28 75.0 71.5 -2
Hilario. Chelsea. 10 80.0 71.4 -9
Reina. Liverpool. 77 72.7 71.0 0
Sorensen. Stoke. 51 72.6 70.7 0
Guzan. AVilla. 28 71.4 702 0
Given. AVilla. 71 70.4 69.9 -1
Ruddy. Norwich. 120 70.0 69.9 +1
Foster. WBA. 96 67.7 68.8 0
Westwood. Sunderland. 33 66.7 68.5 0
Bunn. Blackburn. 14 64.3 68.4 +3
Howard. Everton. 76 65.8 67.9 0
Krull. Newcastle. 88 65.9 67.7 -2
Cech. Chelsea. 66 65.2 67.6 -1
Begovic. Stoke. 53 64.2 67.1 0
Bogdan. Bolton. 24 58.2 65.9 +3
al Habsi. Wigan. 123 63.4 65.4 -1
Jaaskelainen. Bolton. 100 63.0 65.3 -1
Szczesny. Arsenal. 84 60.7 63.6 -1
Kenny QPR. 71 57.8 63.2 0
Robinson. Blackburn. 88 54.6 60.1 0

Joe Hart,comfortingly for England fans grabs the top spot with an adjusted figure that eclipses both Hilario and Lindergaard,who scored better raw save to shot ratios,but were out tested by the Manchester City stopper by factors of 8 and 4 to 1 respectively.

Wolves' Wayne Hennessey has faced 150+ shots on target,the most number so far this season,but still not sufficient to take his ratio completely at face value and so he can't escape the gravitational pull of the league average.Only Norwich's Ruddy barely moves,but that's simply because he is so close to the mean that he has nowhere really to move to.Blackburn would appear to have little to lose by replacing Paul Robinson as his corrected figures barely creep above 60% a line below which few keepers should expect to survive below for very long.

A couple of team mates keep each other company in the revised standings,implying that there is also a team component to the quality of shots a keeper faces.But there is also enough variation between stoppers from the same club,notably in the case of Blackburn,QPR,Sunderland and Stoke to equally imply that goal keeping is a skill that varies between team mates.

So this is a start to tease out the randomness that inhabits all situations and is a bane to all keepers.They more than any position are hostages to short sequences of luck based results where the faulty application of causality brands them either world beaters or dross,when the truth is almost always somewhere in between.


  1. like this stuff.

    not a maths whiz so was wondering if you could tell me how you did that? What I mean is how you took the save% and adjusted it. It is taking the average shots faced by all goalies and making a ratio with it for a certain goalie and then using the average and known save % in some sort of ratio? No idea. :)

    I've got a file of goalkeeper stats (past few years of a La Liga team) ... wondering if you could have a look at it and tell me where I can go from here? How I can incorporate this adjusted save % into it to see which of the 5 goalies has been the best for the team ...


  2. Hi Bart,a quick method to evaluate a keeper's save% is to incorporate a proportion of the league average save% into the calculation and then progressively replace these "average" saves as you get more data on a particular keeper.

    It's a method that ratings sites sometimes use.Say they ask you to rate a film and you give it 10/10,if you're the first and only person to rate it,the average will be 10.But if the site has already included nine average votes of 5/10 then your perfect ten will give it an average rating of 5.5.And that might be closer to the rating it achieves when more real voters give their opinion.
    You can be quite flexible in how you apply the method.

    I'm intending to do a close season post on how many saves you need to look at before they become representative of a keeper's true ability.

    Feel free to email me at mark.taylor0 AT re your Spanish keepers.