World Football Debate: How Does One Evaluate the Worth of a Goalkeeper?

There's a well-known saying in football that says: A good goalkeeper is worth 10 to 15 points a season.

But what exactly determines whether a goalkeeper is good or not?

We've all seen and read a variety of goalkeeper rankings. I did one myself in August on what I thought were the fifty best goalkeepers at the time.

But when you stop and think about it, it's incredibly hard to determine the true worth of a goalkeeper against other goalkeepers in world football. 

For all other positions, statistics can give you a fairly decent representation of how good a player is.

A striker, no matter how much Fernando Torres and Andy Carroll may wish that this was not the case, is judged and ranked by his ability to put the ball in the net—just like a playmaker's value and ability will be evaluated partly on his ability to record a high number of assists.

For defensive midfielders and defenders it gets a bit harder, but a defender's successful tackle percentage and pass percentage should be good enough to give a general idea of how good a particular defender is.

And then of course there are clean sheets, which are less reliable as determinants of individual worth, but still shed some light on the value of a particular player.

But for goalkeepers, everything is subjective.

Some people say that clean sheets are the number one indicator of the quality of a goalkeeper. But clean sheets arguably depend more on the quality of a team's defense than a team's goalkeeper.

A decent goalkeeper will likely rack up more clean sheets than a talented goalkeeper will, if the decent goalkeeper is playing behind a better defense. This is the case for Victor Valdes, whose 11 clean sheets are the best total in Europe and far surpass Iker Casillas' total of seven.

Others will usually point to a goalkeeper's goals against average or the average number of goals a keeper lets in per game. But this stat faces the same flaw that clean sheets face; it is more dependent on the quality of the goalkeeper's team's defense than it is on the goalkeeper's actual talent and ability.

Again we can return to the Valdes-Casillas comparison. Valdes lets in 0.63 goals per game, a miserly amount, which is only bested by the great Gianluigi Buffon, who quietly has reasserted his dominance in Italy by allowing only 0.53 goals per game.

Meanwhile Iker Casillas lets in 0.95 goals per game, an amount which is surpassed by at least five other goalkeepers in the top four leagues (Bundesliga, Serie A, La Liga and EPL) of world football.

The difficulties in comparison have become even more apparent when attempting to compare goalkeepers of teams with a significant gulf in class.

For example, Lazio's Federico Marchetti has averaged 0.74 goals allowed per game, which is better than Iker Casillas' 0.95 goals allowed per game. He has also recorded eight-clean sheets to Casillas' seven—despite having played four less games than Casillas.

Considering the fact that Lazio's defenders are significantly inferior to those at Real Madrid, is it fair to say that Marchetti has been the better goalkeeper this season?

While one could simply concede the argument and say that Marchetti has been indeed the better goalkeeper this season, a point could be made that Real Madrid plays a heavily counter-attacking offensive-style that leaves Casillas regularly exposed at the back. 

But then if we're going to bring a team's style into the equation, doesn't that complicate things even further?

We would have to account for the style of each goalkeeper's team in every power ranking we did, and that would be nearly impossible since there is no statistical way to account for "style" and its impact on a goalkeeper's stats.

I suppose my point is that maybe, just maybe, we as the world football community have had it all wrong.

We've always ranked Iker Casillas as either the first or second-best goalkeeper in the world, and generally believed that goalies playing for the biggest national teams are the best in the world.

While this is sometimes true, I will show in my next article that it does not work as a universal rule.

So as to not end the fun here, I will save my statistical power ranking of the best goalkeepers in the world for a slideshow in the near, near future. But for now, I want you guys to join in the debate.

How do you evaluate the worth of a goalkeeper?

Is it his goals allowed per game average, his clean sheets or a combination of the two that you value above all else?

Is it his saves to games played ratio that does it for you?

Or is it something less statistically measurable that you look for in determining the best goalkeepers in the world?

Is it a goalkeeper's coolness under pressure, and ability to save penalties that you feel is most important, or is it a goalkeeper's experience and ability to regularly claim silverware that convinces you of a goalkeeper's quality?

Please share your input and stay tuned for the Statistical Power Ranking of the World's Best Goalkeepers coming soon to a Bleacher Report slideshow near you.