While analytics in soccer continues to grow each day, most public data available for consumption is limited to mostly the big four leagues in Europe. When first learning about advanced stats and analytics from great sites like Stats Bomb and books such as “Soccernomics” and “The Numbers Game”, I was excited to see how these metrics could be applied to Celtic and the SPFL.
However, I soon learned the stats conscious fan of a Scottish soccer team had only a few bloggers such as Seth Dobson at Fitba Fancy Stats and the mystery man behind (the now defunct) Rangers Report. The fact was, there was little data available for Scottish fans to play with. When I started my blog The Backpass Rule, I wanted to make more data available to fans and share ideas I had about how Celtic and all of Scotland could use that data to help close the resource gap it has with the bigger leagues in Europe.
While Celtic’s resources are much larger than the rest of Scotland, they too have to deal with a resource gap. Look no further than Celtic’s Champions League group to illustrate this, with Barcelona, Manchester City, and Borussia Mönchengladbach having financial resources that Celtic could only dream off. It seems logical that Celtic would look for any edge it can find to try and even the playing field with these mega-clubs in Europe, and I believe that analytics are what they need to use to do so.
With that being said, lets first talk about what the stats suggested about Celtic’s recent 4-1 victory over Aberdeen. One of the most popular advanced stats in soccer today is expected goals (often noted as xG). Advanced Stats guru Michael Caley defines expected goals as: “Expected goals is a method for estimating the quality of chances that a football team creates or concedes in a match. This is the thing I like a lot about xG. It may take a lot of data crunching to create specific xG values, but the underlying idea makes football sense. How many good chances did a team create?” I talked about how I go about measuring expected goals in the SPFL here. Last year, the six teams that finished in the top six of the SPFL Premiership also had the best expected goals differential (expected goals for – expected goals against). The metric is far from perfect, but definitely gives us some valuable information about a team through the course of a season.
When we look at the expected goal output of the Celtic – Aberdeen match at Celtic Park, we find that it was 1.78 – 1.02 in favor of Celtic, while Celtic also out shot Aberdeen 10 to 5 and 5 to 1 in Shots on Target. Statistically, that is a pretty good performance from Celtic. Using simulations and those expected goals and shots data, you would expect Celtic to win 58.3 percent of the time, earn a draw 25.16 percent of the time, and there be an Aberdeen victory just 16.5 percent of the time.
While it is early in the season, Celtic has a positive expected goals difference so far in the season. They also are averaging more shots and shots on target than their opponents thus far, but are only averaging one shot on target more than their opponents. While it is still early to make judgments based on this data, you would like to see Celtic widening that gap as the season progresses, particularly with Celtic’s south-side neighbors scheduled to be at Celtic Park on September 10th. That’s especially true when those neighbors have a better Shots on Target Ratio (0.667 to 0.556) and a better expected goals difference (3.29 to 1).
Leigh Griffiths, Scott Sinclair, and James Forrest all continued their great form thus far in the season, with all three contributing a goal against Aberdeen on Saturday. Those three have scored eight of Celtic’s 10 goals in their first three matches.
James Forrest in particular looks like a whole new player, already surpassing the number of goals scored last season in the league. While Forrest has been outstanding for Celtic thus far, we probably should not expect him to keep us this goal scoring pace. Last season, Forrest had two league goals on 26 shots, 10 on target, and an xG total of 3.15. This is 0.18 Goals per 90 minutes, or a goal every 548 minutes. Thus far this season, Forrest has three goals on nine shots, four on target, and an xG of 0.87.
Expected goals has been found to be a more repeatable metric than goals and Forrest’s expected goals output is on pace to be around 11 if he continues to play this many minutes (and with Forrest’s injury history that is certainly not a guarantee). Currently Forrest is scoring 1.21 Goals per 90 minutes and would be on pace to score around 38 goals. That’s just not likely to happen.
Forrest is greatly over-performing based on his xG numbers thus far and we probably cannot expect him to continue his goal scoring form for the rest of the season. Yet, compared to last year, would any Celtic supporters have believed that James Forrest would be an important part of the club’s success?
Most Celtic supporters would be quite happy with James Forrest if he scored 10-12 goals this season. Even if Forrest regresses, as he is likely to do, a consistent James Forrest offers a lot to Celtic and Brendan Rodgers and furthers the depth on attack for the Bhoys.