It would be tautological to say that you win in football by scoring more goals than your opponent. What is interesting is that scoring more goals and letting in fewer works across games in a season as well, as data from the English Premier League shows.
We had seen an inkling of this last year, when I had showed that points in the Premier League were highly correlated with goal difference (96% R square for those that are interested). A little past the midway point of the current season and the correlation holds – 96% again.
In other words, a team’s goal difference (number of goals scored minus goals let in) can explain 96% of the variance in the number of points gained by the team in the season so far. The point of this post is to focus on the rest.
In the above image, the blue line is the line of best fit (or regression line). This line predicts the number of points scored by a team given their goal difference. Teams located above this line have been more efficient or lucky – they have got more points than their goal different would suggest. Teams below this line have been less efficient or unlucky – their goal difference has been distributed badly across games, leading to fewer points than the team should have got.
Manchester City seem to be extremely unlucky this season, in that they have scored about five fewer points than what their goal difference suggests. The other teams close to the top of the league are all above the line – showing they’ve been more efficient in the way their goals have been distributed (Spurs and Arsenal have been luckier than ManYoo, Chelski and Liverpool).
At the other end of the table, Huddersfield Town have been unlucky – their goal difference suggests they should have had four more points – a big difference for a relegation threatened team. Southampton, Newcastle and Crystal Palace are also in the same boat.
Finally, the use of goal difference is used to break ties in league tables is an attempt to undo the luck (or lack of it) that would have resulted in teams under- or over-performing in terms of points given the number of goals they’ve scored and let in. Some teams would have gotten much more (or less) points than deserved by sheer dint of their goals having been distributed better across matches (big losses and narrow wins). The use of goal difference is a small attempt to set that right.