By Bill Reno (@letsallsoccer)

Last month we took a look at how bad the Goals Against Average (GAA) stat is for evaluating goalkeepers and how the save percentage (S%) is slightly better. To show the inefficiencies of the GAA and S%, I compared them to ASA’s own unique stat of Goals Minus Expected Goals (GmxG). The GmxG looks at where shots are taken, calculating the likelihood of a shot going in from that distance and angle to goal, which ends up telling us if a goalkeeper is giving up too many goals given the circumstances. This is great for a couple of reasons, the main one being that we have a more accurate reading on shots on goal than the ol’ shot percentage. If one goalkeeper consistently sees shots from distance while another is routinely left out to dry by his defense, the GmxG will let us know how many goals each goalkeeper should be conceding even if their SOG and goals are similar. However there are some shortcomings with any stat and the GmxG isn’t any different.

The most obvious limitation of the stat is that it only values shot stopping. It does not take into account how many goals have come from poor distribution, bobbled crosses, or even weak rebounds a goalkeeper has gifted an opposition. For example, Bill Hamid leads the table by quite a margin but it doesn’t take into account his poor rebound against New York earlier in the season. Sure, it’s just one example, but it doesn’t affect a goalkeeper’s GmxG.

The GmxG is also hindered by the fact it needs a large sample size to truly even out. After about one hundred interactions (saves + goals), based off my own observations not anything scientific, it starts to accurately tell of a goalkeeper’s performance. However before then, there are too many variances that can occur to sway the numbers. Aspects the GmxG does not take into account are shot speed, a goalkeeper’s view, and the movement of the ball prior to the shot being taken. All of these affect how difficult it is to save the shot. Perhaps the most annoying usage of an GmxG is after one game. You may apply expected goals to an offense after one game, but there are too many unknowns to judge a goalkeeper on after half a dozen shots.

Lastly, and probably the most damning, goalkeepers are typically benched for mistakes that do not have to deal with shot stopping. Raïs M'Bolhi is a good example of a goalkeeper who struggled in the air and with his own distribution. In fact, he was so miserable that he hardly had a chance to save any shots before he was benched because his mistakes were so numerous. And, of course, let’s not forget Carlo Cudicini. Saving shots is clearly needed in a good goalkeeper, but it doesn’t let us know how else the goalkeeper is helping or hurting their team.

The GmxG is not a WAR (wins above replacement) for goalkeepers. It’s an intense look on one dimension of goalkeeping. Although it is obviously an important one, and one we can learn a lot from, it does have it’s limitations. Is it better than the GAA? Absolutely. Is it better than the Save Percentage? Yes, significantly. Should it be the only thing taken into account in gauging goalkeepers this season? Definitely not. At least for now. The more the stat is smoothed out and missing pieces are marked down, the more confidence we can place in it.