How Luck Integrates With Shots And Our Expected Goal Model / by Harrison Crow

By Harrison Crow (@harrison_crow)

Let's talk shots and luck.

It's obvious that shots become goals by way of two distinct systems of measurement. First quality and then quantity. The better look you get and the frequency at which you take the chance to score implies the amount goals you might score.

This is the very premise by which we created our expected goals model. We've identified certain factors that we've found to create favorable shots. Likewise how often that type of quality shot would reasonable end in a goal.

Now the problem comes with things that we cannot account for in those factors of quality such as defender and keeper placement. A keeper being out of position would obviously improve the quality of the attempted chance and a defender in the right position would lower the quality.

Luck is inevitably part of the system that is active in quantity. Alan Gordon took a very high probability shot against Stefen Frei in the closing seconds of the Galaxy-Sounder match on Sunday evening. He used his most common foot, he took it from close range and he had a large percentage of the goal exposed to him. However, a bad touch created the time necessary for Chad Marshall to get into range to block the shot.

The likelihood of that opportunity becoming a goal is weighted by other scenarios in past instances of other attacks in similar situations and factors. Would Robbie Keane have had so many issues with that shot? How about Landon Donovan or Gyasi Zardes or even any other striker?

The gap between the elite and the good and the average has shown to be much smaller than most would think. Looking at our expected goal table you can see the "G -xG" category which is simply the amount of goals scored subtracted by the amount of expected goals with the variation between the two being around .02 probability per shot. Again, this is not a lot.

Past performance patterns show us G-xG is not necessarily a predictable skill. Eddie Johnson is a great example. With +4 goals in 2012, +1 in 2013 and -1 this year. Keane, a player I think most, if not all, would consider elite within MLS, has shown the same tendencies with 0 G-xG in 2012, +5 in 2013, +2 in 2014. MVP Candidate, Lee Nguyen, posted a -2 G-xG last year and this year is near +9 goals. This is a huge swing and cannot simple be about a jump in a players ability but more about skill WITH luck. We'll get more into this when we talk Jairo Arrieta in the coming days.

This isn't to say that I don't think that skill plays a role. It's obvious fair to say Gordon isn't at the same talent level as Keane, Zardes or Donovan. But that doesn't mean that Gordon is void of talent. I'm not going to say Gordon would make that shot 50 times out of 100. But I wouldn't say that about Keane either. I think there is an edge in Keane's favor, I just believe that edge to be much smaller than most.

It's reasonable to think that luck played a huge part of Gordon not scoring on that shot. I think it's also reasonable to consider this event when evaluating the end game scoreline from either teams perspective. Gordon was in great position to change the game state from a high probability shot. Accounting for that through expected goals gives us insight to how the the striker performed.

These are some things to consider when weighing luck and it's influences on a team's results in scoring goals and allowing them.