Tuesday, 22 December 2015

Home Field Advantage in the 2015/16 Premier League.

The 2015/16 Premier League season has been portrayed as a remarkable one in which the natural order has been upturned. Although as Simon Gleave points out, the only major dislocation from previous years is that Leicester and Chelsea has switched shirts.

Otherwise, the expected strugglers are struggling, the usual title contenders are heading the betting market, if not the actual table and a handful of unfancied mid table fodder has leapt into the top half of the table buoyed by good play, small sample size and a bit of good fortune.

A minor sub plot has been the near equality of home and away results. The raft of away successes early in the season highlighted the apparent supremacy of travelling teams and I suggested that a quarter of the season was insufficient to declare a sea change.

After 170 matches home wins are now back ahead of away victories, but the lead is a narrow one.

Expressed as a success rate, where draws are treated as half a win, away teams are running at 0.49 and home wins, unsurprisingly 0.51.

Historically, the trend is for decreasing levels of home field advantage, although there are inevitably peaks and troughs within the general descent. It therefore makes sense to see if a run of 170 matches where home and away teams came close to parity is unusual in the recent past.

HFA, on the wane.
Success rate for away teams in groups of 170 consecutive matches has ranged from lows of 0.34 to highs of 0.48 since 2002, excluding this season.

2008 began with away teams achieving a 0.46 success rate across the opening 170 matches with home teams outscoring their visitors by just over 0.1 of a goal per match.

But over the season as a whole, home teams were on average superior by 0.32 of a goal per game and away side had a success rate of 0.42.

So evidence for a closing of the gap between host and visitor, but not for parity.

Expected goals for 2015/16 confirm a period of matches where home and away teams have been closely matched, with the former outscoring the latter by just over one tenth of a goal per game.

Simulating all 170 matches results in away sides having an above 0.5 success rate in 16% of the seasons and a success rate as good or better than their actual record in 30% of simulations.

That leaves around 80% of simulated seasons where home teams have the higher success rate and 10% of seasons where that success rate is a healthy 56% or higher.

Again, a continued closing perhaps, rather than an elimination of home field advantage.

The causes of home field advantage, not just in soccer, is not well understood nor universally accepted. Even in the cossetted environment of modern soccer, travel may play a small role, as may crowd support.

And these factors may subtly change from season to season.

However, an important contributor to individual match outcomes is red cards. Eleven verses ten or even nine, is on average a big advantage to the numerically superior team.

Historically away sides suffer more red cards, not particularly because of referee bias, but simply because they are forced into making more tackles.

In 2014/15, Premier League home sides lost 600 playing minutes to red cards compared to 1000 for their visitors. The previous season it was broadly similar, 520 minutes lost by the hosts and 1070 for the away team.

So far in 2015/16 this potent, but relatively rare event is favouring the visitors. Home teams have lost 460 minutes to red cards spread across 11 matches, seven of which have been lost and two drawn.

Away teams have lost 420 minutes.

Home teams may have been unlucky so far based on expected goals in just 170 matches. Refs may not continue to find fault with the home players in a way that is unusual in recent seasons and home field advantage may continue to be a depreciating, but real feature of the current Premier League.

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