I was going to start this post with a story, but I think the headline says it all: there is no such thing as Home Field Advantage in the Premier League this season.
This post was inspired by my eyeball test of MOTSON, my statistical prediction model. My initial reaction was that it was over-estimating the chances of home teams, as most of the “bad” results were from away wins. Inspired by a conversation with @11tegen11, I looked at the overall predictions of my model, and it predicts 47% home victories, which is inline with historical data (last season was ~45%). So if my model is well-calibrated, why does it seem to over-value home field advantage? Simple: home-field advantage doesn’t exist this season.
As of this morning, we had 167 fixtures played. The home team won 62 times, drew 46 times, and lost 59 times. First cut at the evidence: the number of wins is basically equal to the number of losses. The home team wins almost as frequently as they lose.
The next thing I did was look at the points-per-game (PPG). Home teams earn 1.39 PPG, while away teams earn 1.33 points PPG. These numbers look basically the same, and a statistical test confirms that they are indistinguishable from each other (p = 0.705).
The next step was to compare this to last year. A quick look at least year’s results show the home team won ~45% of the time, drew 25% of the time, and lost ~30% of the time. This comes out to 1.6 PPG for the home team. So again, I ran a statistical test and found that this number is statistically distinguishable from this season’s results (p < 0.05).
As the barplot shows, the difference between Home PPG in 2015-16 is outside the 95% confidence interval, while Away PPG and the PPG where win, lose, and draw are assigned (1.32) equal likelihood are well within the range. In statistical terms, we can say that Home PPG 2015-16 is distinguishable from 2014-15, but not the other two categories.
The final test was to test whether a randomly generated series of results was statistically distinguishable from this season. So I simulated 10000 “seasons” up to this point with an equal probability of win, lose, and draw for each fixture. I then calculated the PPG for each of these seasons, and compared those randomly generated seasons to the current one. Of the 10,000 simulated seasons, only 140 were statistically distinguishable from the current one.
The blue highlighted part of the graph represents seasons that are indistinguishable from this one, while the much smaller red part represents seasons that are statistically distinguishable from this one.This means that if the “true” probability of winning at home was equal to the probability of losing at home, we’d randomly see a season that didn’t look like this one only 1.4% of the time. Simply said, the trend documented in this piece by Oliver Roeder and James Curley has continued and home field advantage in the EPL has disappeared this season.
I have no way of saying whether this season is an anomaly or whether the current trend will continue. It is significantly different than last year, but given the trend it’s hard to say whether last year was the outlier or this one was. Only time will tell, but what we can say is that home field advantage is worth significantly less than it was last year. That is to say it had value last year, and we can’t tell any effect at all this season.
Author’s note: I aimed the statistical discussion here for a general audience with some knowledge of principles of stats but without knowledge of the different kinds of t-tests. Replication R code and a more technical version will be available as soon as I get around to writing it.