There is no position on the soccer field where performance is defined with as much subjectivity as goalkeeper, a strange state considering the role has one clear, chief responsibility. As the last line of defense, the goalkeeper’s tasked with preventing, well, goals. While strikers can also be judged by the opportunities they create, and a defender’s distribution can augment their value, goalkeepers live in the most bottom-line of worlds. Good ones just don’t allow a lot of goals.
That makes it easy to reduce goalkeepers to one number – how often they allow goals – though let’s not be all strawman-y about this: Most people’s evaluations are more nuanced.