An ‘expected goals model’ is a method of estimating chance quality more precisely than mere shot numbers, looking at the historical likelihood of a shot going in based on factors like shot location, whether or not the shot was a header, and the angle from goal. It correlates strongly with goals year on year, and provides a useful basis to compare team strength.



Despite this long-term correlation, teams will often over or under perform their expected goal difference within a season. An example of this is Swansea City in the Premier League last year, who finished 8th but had drastically overachieved given the chances they created over the campaign. This time around, they have faced a relegation battle.



In this sense, we can conceptualise ‘luck’ as how likely something would be to happen again. In footballing terms, expected goal over-performance is often driven by unsustainable finishing rates (above long-term trends) for you, as in your attack scoring more of its chances, or against you, as in your defence conceding less of its chances.



For the opening of this two-part look into which teams have been lucky in Serie A, we will be looking at teams who have over-achieved their expected goals difference.



We can measure this overachievement by calculating the difference between a team’s actual goal difference and its expected goal difference:



Over-Performance = (Actual Goal Difference) – (Expected Goal Difference)



If a team’s ‘over-performance’ is greater than 0, they have been ‘lucky’, and if it is less, they have been ‘unlucky’. Here, we will only look at teams for which it is positive.



Roma have been the luckiest team in the league this season, and by quite a margin too: at the beginning of March, Napoli had exceeded their expected goal difference by about 10 goals; Roma had outperformed theirs by nearly 17. In terms of the average expected goal difference of their matches, Roma are the 4th best in Serie A, behind Fiorentina, and so this extra luck helps explain how they are nonetheless 5 points ahead of them in 3rd.







Bologna, currently 10th in the league, are the next biggest over-performers, closely followed by Sampdoria, who are 15th in the league. Had Sampdoria not over-performed their expected goal difference this season, they would likely have been more caught up in the relegation battle.



Through separating attack and defence, we can see whether a team’s over-performance was driven by them scoring more or conceding less than the long-run trend.







Of the league’s over-achievers, Roma’s attack has outperformed its expected goals the most. They have scored as many goals as Napoli, and so are the joint top attack in the league, but in terms of chance creation they only rank 3rd by my expected goals model. Napoli’s attack has over-achieved too, but they also create more chances than Roma and Juventus, and this is likely to be more sustainable in the long-run.







Looking at defence, Inter clearly lead the way in terms of conceding less goals than they should have. This is not a good sign, and if Mancini’s team want to launch a legitimate challenge to the top of Italy’s hierarchy next season, they will need to make sure they concede less chances – their expected goals against is 11th worst in the league. Interestingly, Fiorentina’s defence has shipped noticeably more goals than it might normally have, offsetting some of their offensive over-performance and probably costing them a chance at the last Champions League spot.



As a team, over-performing in a season isn’t necessarily a bad thing. If you can register where you have and improve in certain areas, you can consciously ride a wave of good fortune and try to protect yourself from that relationship inverting. Teams like Roma should be especially wary of their marked over-performance, and make actionable changes to avoid experience the downturn of throwing up the same chance creation without the lucky finishing that blessed them this season.



Bobby Gardiner @BobbyGardiner











