Nine games into the current Barclays Premier League season, Southampton have conceded just three goals. Currently leaking goals at a rate of one every three games, the Saints are currently on track to eclipse Chelsea's 2004-2005 Premier League record of conceding only 15 goals in an entire season.
Southampton have an exceptionally strong defence, but is it on a par with the best defence in the Premier League's history? Probably not. Unless we believe that Southampton could theoretically be making a legitimate bid in the Champions League (given the opportunity), we're forced to admit that their defensive record is currently operating at an unsustainable rate. While Southampton's defenders have been worth their weight in gold in Premier League Fantasy Football teams, selecting them has, in all likelihood, become a game of chicken. At some point, they must begin to regress toward the mean.
There are two ways to avoid conceding goals: prevent opportunities, or effectively mitigate opportunities when they inevitably arise.
Southampton have been good at preventing opportunities. They have conceded only 87 shots this season, tied with Tottenham. Only Manchester City has conceded fewer, with 85. This is exceptional, even when considering the Saints' comparatively easy schedule over their first quarter of the Premier League season.
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*Note: This analysis does not include own goals and penalty kicks.
But, shot volume doesn't tell the whole story. Using OptaPro's shooting model (an updated version of this), which is based on a handful of different measurements including location and pattern of play, we can estimate the probability of a shot resulting in a goal. Southampton, while conceding very few opportunities, concede relatively dangerous ones. However, many of these dangerous opportunities have gone awry, conveniently preserving the narratives surrounding the Southampton defence.
Quantitatively determining how well a team mitigates a high-quality opportunity is a bit more hairy. Looking at the volume and quality of shots that Southampton has conceded, they have conceded 4.6 fewer goals than predicted by the model – currently the largest positive deviation in this year's Premier League (Conversely, it also provides a glimmer of hope for Sunderland, as their opponents' conversion rate seems similarly unsustainable). Deviation from this model can be attributed to two things: un-modelled bias and luck.
One thing this model assumes is that the player attempting each shot will convert his chance at a league-average proficiency. Of the elite teams that Southampton have faced in their first nine matches, it's only included a struggling Manchester United and a Suarez-less Liverpool. This strength-of-schedule imbalance undoubtedly adds some bias in favour of Southampton's defence.
No model is perfect, including this shooting model, so evaluating model residuals should only be done with an adequate understanding of the model's limitations. Sometimes luck outweighs the bias, but other times it may not. The distribution of the effect of un-modelled bias and luck on model variance is unlikely to be constant.
This analysis does not prove that Southampton have been exceptionally lucky, just like it fails to prove that Southampton's defence is as good as Chelsea's during their record breaking 2004-2005 campaign. But, unless Southampton are consistently repelling opposition chances in a way that's so revolutionary that it entirely eludes our shooting model, it's unlikely that Southampton has achieved this outlying defensive efficiency purely on their own accord, and the stage is set for them to start conceding goals at an increased rate.