This brief post builds slightly on the excellent work published by @Colinttrainor on his site statsbettor.
I hope he won’t mind my use of the data to try and highlight a few additional points of discussion…
I took his image, re-ordered it by actual goals conceded, and added rankings for his defined prime and secondary shots volume to attempt to review how well it explains the actual number of goals conceded by each team last season:
Spurs actually rank #1 in terms of the volume of shots in prime and secondary positions allowed but 8th in terms of goals conceded. West Ham, on the other hand, allowed the second-worst number of shots in prime and secondary positions but managed to finish mid table in terms of goals conceded. But is that luck or skill?
Defensive pressure on the shooting player, goal-keeping ability or plain old luck could feasibly be reasons for this.
At the bottom of the above table, I input an expected average assumed goals per shot to try and attribute how much each shot allowed might be worth to the opposing team – this wasn’t scientifically calculated, just a ball park set of figures to infer the goals we might anticipate that each team would concede if the season were replayed (several times). I’m suggesting that a shot in the prime area is worth something like 0.25 goals, 0.1 goals in the secondary area and then 0.05 and 0.04 for marginal and poor areas respectively.
The result is below:
Arsenal, West Brom and QPR are closest to the expected goals tally, suggesting perhaps that luck didn’t play much of a part in their defensive results last season.
And a simple indication of probability on the far right column suggests that, assuming skill does not play a part in under or out-performing the expected result (I think it certainly does to some extent), then Wigan, Southampton and Newcastle were the most unlucky in terms of goals conceded whilst West Ham, Stoke (who else?) and Sunderland were the luckiest defensively in the league last season.
Of the teams in the top half, Liverpool and Spurs both conceded more goals than expected (+5 and +10 respectively) which suggests a need for improvement if we cannot apportion their underperformance to luck.
**NB @colinttrainor ‘s work strips out headed shots, but my additions are based on all goals conceded. A relatively small but not insignificant note to the above.**