Monday, January 4, 2021

COVID and soccer home team advantage - winning less often

Home advantage

Is it easier for a sports team to win at home? The evidence from sports as diverse as soccer [Pollard], American football [Vergina], rugby [Thomas], and ice hockey [Leard] strongly suggest there is a home advantage and it might be quite large. But what causes it? Is it the crowd cheering the home team, or closeness to home, or playing on familiar turf? One of the weirder side-effects of COVID is the insight it's proving into the origins of home advantage, as we'll see.

(Premier League teams playing in happier times. Image source: Wikimedia Commons, License: Creative Commons, Author: Brian Minkoff)

The EPL - lots of data makes analysis easier

The English Premier League is the world's wealthiest sports' league [Robinson].  There's worldwide interest in the league and there has been for a long time, so there's a lot of data available, which makes it ideal for investigating home advantage. One of the nice features of the league is that each team plays every other team twice, once at home and once away.

Expectation and metric

If there were no home team advantage, we would expect the number of home wins and away wins to be roughly equal for the whole league in a season. To investigate home advantage, the metric I'll use is:
\[home \ win \ proportion = \frac{number\ of\ home\ wins}{total\ number\ of\ wins}\]
If there were no home team advantage, we would expect this number to be close to 0.5.

EPL home team advantage

Let's look at the mean home win proportion per season for the EPL. In the chart, the error bars are the 95% confidence interval.
For most seasons, the home win proportion is about 0.6 and it's significantly above 0.5 (in the statistical sense). In other words, there's a strong home-field advantage in the EPL.

But look at the point on the right. What's going on in 2020-2021?

COVID and home wins

Like everything else in the world, the EPL has been affected by COVID. Teams are playing behind closed doors for the 2020-2021 season. There are no fans singing and chanting in the terraces, there are no fans 'ohhing' over near misses, and there are no fans cheering goals. Teams are still playing matches home and away but in empty and silent stadiums.

So how has this affected home team advantage?

Take a look at the chart above. The 2020-2021 season is the season on the right. Obviously, we're still partway through the season, which is why the error bars are so big, but look at the mean value. If there were no home team advantage, we would expect a mean of 0.5. For 2020-2021, the mean is currently 0.491. 

Let me put this simply. When there are fans in the stadiums, there's a home team advantage. When there are no fans in the stadiums, the home team advantage disappears.

COVID and goals

What about goals? It's possible that a team that might have lost is so encouraged by their fans that they reach a draw instead. Do teams playing at home score more goals?

I worked out the mean goal difference between the home team and the away team and I've plotted it for every season from 2000-2001 onwards.
If there were no home team advantage, you would expect the goal difference to be 0. But it isn't. It mostly hovers around 0.35. Except of course for 2020-2021. For 2020-2021, the goal difference is about zero. The home-field advantage has gone.

What this means

Despite the roll-out of the vaccine, it's almost certain the rest of the 2020-2021 season will be played behind closed doors (assuming the season isn't abandoned). My results are for a partial season, but it's a good bet the final results will be similar. If this is the case, then it will be very strong evidence that fans cheering their team really do make a difference.

If you want your team to win, you need to go to their games and cheer them on. 

References

[Leard] Leard B, Doyle JM. The Effect of Home Advantage, Momentum, and Fighting on Winning in the National Hockey League. Journal of Sports Economics. 2011;12(5):538-560.

[Pollard] Richard Pollard and Gregory Pollard, Home advantage in soccer: a review of its existence and causes, International Journal of Soccer and Science Journal Vol. 3 No 1 2005, pp28-44

[Robinson] Joshua Robinson, Jonathan Clegg, The Club: How the English Premier League Became the Wildest, Richest, Most Disruptive Force in Sports, Mariner Books, 2019

[Thomas] Thomas S, Reeves C, Bell A. Home Advantage in the Six Nations Rugby Union Tournament. Perceptual and Motor Skills. 2008;106(1):113-116

[Vergina] Roger C.Vergina, John J.Sosika, No place like home: an examination of the home field advantage in gambling strategies in NFL football, Journal of Economics and Business Volume 51, Issue 1, January–February 1999, Pages 21-31

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