As Atlanta United, and MLS, develops deeper knowledge and use of data analytics, we’ll be taking a look behind the curtain at some of the specific stats the club values. In our new weekly content series, we examine one metric provided by StatsBomb that may not stand out on a first watch, but upon closer evaluation, is a useful tool to measure the team’s performance.
The stat:
Post-Shot Expected Goals (and Goals Saved Above Average)
What it measures:
A goalkeeper’s ability to stop high probability shots.
Atlanta United has played four matches so far this season, a small batch of data to mine from, and the most recent match at Toronto FC was arguably the least like the previous three. For one, Atlanta United was missing seven players, including reigning MLS Young Player of the Year Thiago Almada, the club’s leading scorer this season in Giorgos Giakoumakis and starting center back Stian Gregersen. The conditions in Toronto were challenging too with temperatures dipping below freezing and ice reported on the field.
The 5-Stripes lost the match 2-0 behind several players making their first starts of the season. And on paper, Toronto FC certainly was the better side on the day. Atlanta United was outshot 21-8, won fewer duels and finished with a mere 0.8 expected goals. Toronto had more shots on goal (9) than any other opponent Atlanta United has faced this season. The second most was five in Atlanta’s 4-1 win over New England Revolution on March 9.
However, one stat Atlanta United excelled in was on the defensive side. In a match that called many young players to see more playing time, it was Atlanta’s seasoned goalkeeper that was a bright spot on the score sheet.
Starting goalkeeper Brad Guzan made seven saves on the night according to StatsBomb, two off from tying a career high. He’s made eight saves or more four times in his Atlanta United career.
Guzan’s strong showing Saturday was clear in the post-shot expected goals category. To understand this metric, we first need to look at what happens when a shot is taken. When a player takes a shot, the shot is assigned two values: expected (or pre-shot) goal and post-shot expected goal. We’ll be looking at the metric of post-shot expected goals because while this also determines how good a shot is, it also shows a goalkeeper’s ability to stop good shots.
The value of a post-shot expected goal measures the probability of a shot resulting in the goal after the shot has been taken. The placement, the quality of the finish and the difficulty required to make a save determine the post-shot expected goal value. For example, a firmly hit shot placed well in the tight window of the upper corner, arguably a challenging stop for any leaping goalkeeper, might be valued at 0.99. A shot that is hit well but is placed right down the middle of the net might be valued at 0.5. A shot that misses the goal entirely is assigned a post-shot expected goal value of zero.
Let’s take a look at an example from the match. In the 71st minute, Toronto scored their second goal of the night when Prince Owusu cleaned up a rebound in front of goal. But let’s take a look at the initial shot that led to the rebound in the first place.
Toronto midfielder Alonso Coello received a pass inside the 18-yard box on his right foot. The area was crowded, and Coello was surrounded by a few Atlanta United defenders, so his shot very easily could’ve been deflected. In this situation, the pre-shot expected goal value of the shot was 0.06.
Coello made contact and placed a shot on target that was heading toward the bottom corner. The probability of the shot turning into a goal increased with how the player struck the ball. It was a good shot, and the post-shot xG was 0.85. By that number, 8½ times out of 10 that shot results in a goal.
In this case, it did not result in a goal – thanks to Atlanta United’s big, bald wall. A value of 0.85 post-shot expected goals shows that this was an extremely difficult shot for a goalkeeper to stop. Yet, Guzan reacted quickly. He dove and with his right hand blocked Coello’s shot. Statistically, of the seven saves Guzan made on the night, this save on Coello’s shot was the most difficult for him to make.
In this situation, Guzan would be awarded a +0.85 in the goals saved against average category, because he stopped a shot with a high probability of going in the net. Guzan and Atlanta United conceded two goals on Toronto’s 2.44 expected goals. Toronto finished the night with 3.1 post-shot expected goals, which means Guzan saved +1.1 goals more than average.
What does this mean exactly? To end up in the green against a team that was producing a lot of quality shots on goal demonstrates that Guzan handled the situation well – and that Atlanta United’s goalkeeping likely saved the team from giving up at least one more goal. That type of solid goalkeeping could indicate Atlanta United is performing well in the category compared to other clubs. Guzan has started every match for Atlanta United this season and ranks in the top-5 in MLS in post-shot expected goals, according to StatsBomb.
Let’s look at one more example from the match to drive this point home. In the 43rd minute, Guzan came up with another save. Toronto FC curled a cross into the box right over the leap of Atlanta United center back Derrick Williams. Toronto’s Deandre Kerr rose to put a firm header on frame.
Shots don’t get much closer than this one. The header arrived right on the edge of the six-yard box. The close range gave the shot a pre-shot xG value of 0.21, which means two times out of 10 that shot would go in. But because Kerr used his head, that created a little bit less certainty and control with his shot. The calculated post-shot xG for Kerr’s header was 0.23, not nearly as high as Coello’s shot later in the match.
Still, the closeness to goal created a dangerous situation for Atlanta United. Guzan's reaction was one of his quickest of the night. His block cleared Kerr's header over the crossbar and prevented Toronto FC from scoring what very nearly could’ve been their second goal of the night.
Guzan's play at the goalkeeping position has been good to start the season. Even after conceding twice to Toronto, Atlanta United still ranks tied for third in MLS in fewest goals against (4). Guzan has faced 19 shots this season and recorded 15 saves. His quality of play will be put to the test Sunday as Atlanta United hosts Chicago Fire, a team that has scored nine goals this season. Atlanta United will have most of their players back from international duty, and this one will be played at Mercedes-Benz Stadium, the site of Guzan’s last clean sheet on March 17.
Atlanta United's Data Scientist & Analyst Arjun Balaraman contributed to this story. Telestration by Khoury Kennedy, Atlanta United's Motion Graphics Producer and Video Editor.
