So this image is floating around on Reddit Soccer and I thought it would be instructive to look at its findings in context to illustrate why we should be careful not to draw easy conclusions.
Obviously this whoscored.com diagram seems to demonstrate that Jose Mourinho is dead on the money in rating Diego Lopez above Iker Casillas. But we should look at each category in turn to see why this may not be as conclusive as we might think.
For one, we don’t have a fair comparative sample size, correcting for opponent strength. We don’t know whether any of the indicators on this comparative chart are meaningful after either a 12 or 19 game sample. For all we know, the numbers are random noise, not reflective of underlying skill.
Take saves per game—perhaps Real Madrid’s defenders had a single bad game which drove up the number of shots, and the resulting number of saves. This isn’t a useful statistic.
The save percentage is a little different, though the sample size is small and we do know that save and shooting percentage are largely a product of random in game luck, which makes a little sense, considering the relative rarity of goals, and shots on goal. Still, simply pointing to a higher number as an absolute measure of quality would be a fool’s errand.
Moving down the list, crosses claimed—we would need to know the total number of crosses in the area to get a sense of fair comparison (i.e. percentage of crosses claimed), and even then, we have no idea whether this metric is a function of luck or opponent strength or tactical formation.
Accurate long-balls per game—accurate in what sense? Did he pick out a target from distance? Or was the long-ball merely better claimed by one of his team-mates? And again, all the above rules apply.
I know pointing these things out is a bit like popping a balloon, or shooting a fish in a barrel, but simply collecting a grab bag of numbers with the veneer of meaning is not a good objective measure of quality. It’s actually really goddamn difficult to determine via player stats just what makes a good goalkeeper good.