The football analytics community most recent civil war with itself has been on the subject on finishing skill. Whether or whether not a player can improve, do something, be better at finishing than fellow pros has been a hot topic. The idea of expected goals heavily relies on there not being a discernible difference in finishing ability amongst teams in a league, since all chances are judged the same in a model no matter who takes it.
One of the things to like about expected goals is that it does not treat every chance the same, such as a a stat like conversion rates (goals/shots taken) does. Teams may have a higher and lower than average conversion rate at points of the season, but usually teams conversion rates regress to the mean over the season.
Of course, Scotland is a bit different than most leagues. Friend of the blog Seth Dobson compiled 10 years worth of shots in the SPFL and found that Celtic consistently had a higher conversion rate than the league average. However, even Celtic has a mean that it regresses to. Since their 5-1 Glasgow Derby victory, Celtic have not lost and only drawn once in the league. Yet despite their impressive form, Celtic had a conversion rate of 25% before the derby and now have a conversion rate of below 19%. Regression happens to everyone.
After being inspired (well, taking his idea and applying it to the SPFL) by the EPL stats blog “Every Team Needs a Ron” (as in Harry Potter), I decided to compare each club’s conversion rate and the conversion rate of their opponent. In his blog, Mark from ETNAR says “As a rough guide, anything within +4% to -4% conversion difference seems like noise that is hard to discern meaning from.” Keeping this in mind, we see most of the league is within that range, with two outliers of Celtic and Ross County.
I thought it would be interesting to compare these conversion rate differences to each club’s expected goal difference, as well as their goal differential. A club’s spot on the graph is based on their expected goals and conversion differences, as well as the size of their circle is based on their goal difference.
Celtic, Aberdeen and Hearts all have positive conversion rates and expected goal differences. It is no surprise that these clubs are in first, second, and fourth in the league.
Dundee and Ross County look marooned on the relegation islands with their negative conversion rate and expected goal difference. County particularly look in trouble with their conversion rate falling outside of that + or – 4% conversion rate difference that we could chalk up to noise. Blog favorite Liam Boyce has missed some time with a knee injury, so his return will be a welcome site for County, but Boyce will need some help to pull County out of a relegation fight.
The only team that has the dubious distinction of a positive expected goal differential and a negative conversion rate is Rangers. They currently sit in 5th place in the league, well below the standards their fans demand. Much of the blame for the Bear’s place in the table has fallen on the shoulders of their defense. The back line has been a middle of the league defense, with the 4th best expected goals conceded total and the 6th best conversion rate conceded.
While the defense has been ok but not spectacular, I might suggest much of Rangers issues have stemmed from their attack. With the fourth best expected goals for in the league, Rangers should have a conversion rate a lot better than just above 8%. Have Rangers been unlucky or are their attackers not good enough? I would say a little of both.
Harry Forrester has the highest xG total at the club, and is 8th in the entire league, but only has 4 shots on target all year. Rangers leading scorer is 36 year old Kenny Miller. You would expect some more bounces to go Rangers way as the season goes on, but even if their finishing moves up towards the mean, it likely still will not be enough to be a title contender. It could see Rangers challenge the likes of Aberdeen and Hearts for 2nd though.
Finally, looking at the top left quadrant of the graph we see Kilmarnock and St. Johnstone in what I like to call the “lucky quadrant”. St. Johnstone are on the edge of the “lucky quadrant”, with only a -0.53 xG difference and conversion rate difference of 3.14. Much of this is thanks to the wonderful play by their young keeper Zander Clark who is the owner of a 88% save rate. Tommy Wright has managed to buck the stats for his entire tenure with the Saints and this year is no different, though some of the Perth clubs stats are still good this year.
Meanwhile Kilmarnock were many’s pick for relegation this year and their xG total is of a club fighting relegation. Yet, Killie currently sit in 7th place in the table, seven points safe. Killie’s place in the table has much to do with their conversion rate of over 16%, the 2nd highest in the league behind Celtic. Souleymane Coulibaly is good, but he’s not that good. Expect Kilmarnock’s finishing to regress and likely will coincide fall down the table, but with a 7 point cushion, Killie could prove the pundits wrong again and avoid relegation.