Are clean sheets a dirty metric for rating goalkeepers?

3Apr 2013

For soccer teams, goalkeepers are a vital last line of defence. The difference between having a good keeper between the sticks and having a bad one could be tens of goals a season. But how can you identify which keepers are the best – and how does this affect your betting?

The most popular metric for measuring goalkeeper success is “clean sheets”. You hear about it almost every week from football pundits, and while not letting the opposition score is important, it’s not the ultimate indicator of a goalkeeper’s ability.

Let’s compare No.1′s from two very different sides – Man City and QPR. Joe Hart (City) has an impressive 13 clean sheets – the most in the league – while Julio Cesar (QPR) has just 5. If this were the only metric we examined, it would be obvious the Hart was the stronger keeper.

A slightly more accurate statistic would be if we factored in the number of games each player has played – 29 vs. 21. Now Hart’s achievements seem even more impressive. The Laser Blue ends games with a clean slate 44.8% of the time, while Cesar manages it on just 23.8% of occasions.

This is only part of the picture, however, because in those 21 games, Cesar made 84 saves – far more than Hart’s 49. He’s also only conceded five more goals than the City star, despite playing for relegation-battlers QPR.

Religiously following clean sheet charts totally negate other, even more important statistics, and can cause people to overvalue the ability of goalkeepers – particularly at top clubs.

The table to the left shows that seven of the 10 keepers with the best clean sheet percentage come from the “big” Premier League clubs. Only John Ruddy (Norwich City), Asmir Begovic (Stoke City) and Simon Mignolet (Sunderland) stand out – potentially something to note when they’re on the field.

Better indicators? Shots per goal & saves per goal

Although less glamorous and more difficult to come by, researching the shots per goal conceded and saves per goal will give you a better picture of a goalkeeper’s ability.

Shots per goal conceded is a good indicator of both how well a goalkeeper can stop shots, as well as how good he is at organising his defence to minimise optimal scoring chances.

There is potential for biases for both good and bad teams in this indicator – good teams might be more adept at preventing clear-cut shots (increasing the number of shots per goal), while at the other end of the spectrum, worse teams might face more opportunistic strikers, hoping to exploit a worst team with speculative shots.

The latter reason might explain why the table to the left is dominated by teams from lower down the table – Swansea, WBA, Stoke, Southampton, Sunderland, QPR and West Ham. Even more interestingly, it shows that Paulo Gazzaniga, Southampton’s No. 2, actually has the seventh best performance in the league.

Saves Per Goal

The most objective indicator of goalkeeper performance could be saves per goal. This eliminates the two biases for shots per goal, as the No. 1 has to physically block the ball from going into his goal to be counted.

Interestingly, it’s actually Gerhard Tremmel of Swansea City that leads the rankings with 4.7 saves per goal. Two of the goalkeepers widely considered the best in the league – David De Gea and Petr Cech – came in second and third at 3.9 and 3.6 SPG each.

How can this help soccer betting?

Most bettors rely on subjective assessments of goalkeepers when determining their ability, which can be detrimental in betting. For example, it’s a natural assumption to assume that a team’s second-choice goalkeepers will be less sure of himself between the posts than the first-choice, and therefore when he’s playing the team should concede more.

In many cases, however, this hasn’t proven corrected. Looking at the saves per goal data, Southampton’s first choice keeper Artur Boruc is actually bottom of the table at just 1.4. The two other men who have represented Southampton between the sticks – Kelvin Davis and Paulo Gazzaniga – are rated at 1.5 and 2.7 saves per goal respectively.

Therefore rather than expecting a worse performance without Boruc, bettors might be more accurate in assuming no difference, or even a better performance. If we were just to examine the clean sheet information, however, Boruc has three compared to just one for Gazzaniga.