Wednesday, 28 December 2011

Home Advantage in the Premiership.

Home field advantage has been an ever present,if declining factor in the top English league since it's inception in 1888.The benefit derived from playing on your home turf back in the 19th century was at times three times as great compared to today's value.Home teams outscored away teams by an average of 0.9 of a goal in the first season of league football and three seasons later the figure had risen to well over 1.This elevated figure from the past no doubt reflects the arduous difficulties involved in travelling relatively large distances to fulfil the fixtures.The amount on average by which home teams currently outscore their visitors is currently of the order of 4 tenths of a goal,although there are fluctuations from season to season.

The easiest way to demonstrate the importance of home field advantage is to see how the chances of two equally matched teams changes if we move the game from a neutral venue to a partisan one.Two typical,equally matched EPL sides at a neutral ground would each have around a 36% chance of winning the game within 90 minutes.However,if we shift the venue,the home side now has around a 46% probability of winning outright compared to the visitors 27% chance.

This is a mighty increase in pre game win probability and it immediately begs the question are some teams able maximize the advantage of playing at home and consistently improve on these already impressive numbers.In short do some teams specialise in winning at home.

Firstly,here's a quick and dirty method to determine home advantage (skip straight to the table if the method becomes too convoluted).Ideally we would need two teams to play a large number of games at a neutral venue,followed by a large number at each other's respective grounds.The change in goal difference from neutral venue to Team A's ground would represent Team A's home advantage and similarly with the change in goal difference from neutral venue to Team B's ground.In reality over one season the closest we can get to this ideal is one team's goal difference against the rest of the league at home and on the road.If we assume that the average home advantage of this team's 19 opponents is going to be reasonably close to the average home advantage for the league as a whole,we can subtract this figure from the spread to give a home advantage,in goals for that particular team in that particular season.

As usual an example to make the situation clearer.Chelsea's average goal difference per game at home last season was +1.37 goals,while on the road it was +0.53 goals.That's a range of 0.84 goals and that figure can be considered to represent the home advantage of every team in the league bar Chelsea plus the average home advantage for Chelsea alone.We know that the average home advantage for all teams last season was 0.42 goals and we can hope that 19 games involving 19 of those 20 teams will average out to close to that figure.So if we subtract 0.42 from 0.84 we get Chelsea's average home advantage for 2010/11 as being equal to 0.47 goals.That's slightly above the league average figure and looking at Chelsea's win/loss home and away split the numbers appear consistent.The success rate for home teams in 2010/11 was 61% greater than that of away teams and Chelsea's home success rate was 63% higher than it's away success rate.

If we do the maths for every team during the 2010/11 season we can get an illustration of how much better or in some cases worse teams played at home compared to their performance on the road assuming that their 38 opponents combined to give performances that were near to the league average.

Home Advantage in the EPL 2010/11.

TEAM. Home Advantage in Goals per Game.
Man Utd. 1.37
Liverpool. 1.26
Newcastle. 1.16
Stoke. 1.10
Blackburn. 0.95
Aston Villa. 0.95
Bolton. 0.89
Wolves. 0.68
Man City. 0.52
Chelsea. 0.47
Birmingham. 0.42
West Brom. 0.42
Tottenham. 0.31
West Ham. 0.31
Everton. 0.16
Blackpool. 0.10
Fulham. 0.05
Sunderland. 0.00
Arsenal. 0.00
Wigan. -0.53

The first thing to notice is that there is a large variation around the average value of 0.42 goals for the league,Manchester United appear to have a massive preference playing at home compared to on the road,while near neighbours Wigan actually performed better away from their home turf.This quite naturally can lead to the impression that better teams manage to muster larger than average home advantages.However,better than average teams such as Spurs and Arsenal appeared to have below average home field advantages last year and lesser teams such as Stoke,Blackburn and Bolton made good use of their home stadium.So it's much more likely that we are merely seeing perfectly natural seasonal variations probably caused by a variety of random events.The values recorded for last season are perfectly acceptable as descriptive statistics for how the likes of Man Utd and Wigan performed at home compared to their away matches during the 2010/11 season,but it provides little in the way of predictive value for future seasons.Wigan for example had a fairly normal home advantage in 2009/10.

To get a better idea of how various team's home and away splits pan out it's necessary to look at more than a season's worth of results.So the next table shows the EPL home advantage for teams who have spent 10 or more seasons in the top flight since 1992.

Home Advantage for EPL Teams 1992-2011.

TEAMS. Home Advantage in Goals per Game.
Newcastle. 0.68
Liverpool. 0.59
Fulham. 0.52
West Ham. 0.49
Man Utd. 0.48
Tottenham. 0.47
Blackburn. 0.46
Man City. 0.41
Sunderland. 0.40
Chelsea. 0.39
Everton. 0.38
Bolton. 0.34
Leeds. 0.34
Middlesbro' 0.34
Arsenal. 0.28
Aston Villa. 0.26

 We now see the seasonal variations disappearing to a great extent and the majority of the teams over a longer timescale have home advantages that are within about a tenth of a goal of the league average of 0.44 of a goal over that time span.There are a few teams that do seem to be slightly more  potent at home compared to away than is typical for EPL teams as a group and also teams such as Arsenal who consistently struggle to make the most of home advantage,but the extremes we saw when home advantage was viewed over a single season are no longer present.

We've seen here that over the relatively short time span of a single season,the partly random nature at which goals arrive during a game can influence more visible global game statistics and it's immediately obvious that Wigan's extremely poor home field advantage in 2010/11 coincides with an abnormal first goal of the game home and away split.On average teams score the game's first goal more often at home than they do away,but through random chance Wigan scored the first goal in just 33% of their home games where a goal as scored.By contrast they notched the first strike in just over 50% of their away games and this abnormal split mirrored their much better form away from home compared to that at the DW Stadium.So it appears that whatever factors allow teams to perform better at home than away on average over the long term can be countered in the short term by other more random factors such as goal scoring times.

To confirm this suspicion,if we plot each EPL team's home field advantage for the 2010/11 season against the differential between those teams scoring the game's first goal at home and on the road,we find that teams with apparently large HFA also score first at home proportionally much more often than the league average.Conversely,teams with very low or even negative HFA scored the first goal away proportionally more often than at home.This leads to the conclusion that short term,abnormal home advantages are likely to be partly the product of the partly random rate at which first goal are scored,they certainly aren't teams specific trends that should be given total credence.In short teams with large home advantages in one season owe that record partly to them getting lucky with how many times they scored first at home compared with on the road.

Seasonal Graph illustrating the increased Home Field Advantage enjoyed by team's who score a higher proportion of  1st goals at home compared to away.
  Having established the likelihood that random factors can skew an individual team's HFA in the short term we should now realise that if we are to make sweeping generalisations about home advantage we must look at much longer term team sample sizes or use the entire season's results.

One area ripe for investigation is how the amount of home field advantage is distributed throughout a game.Do home teams come flying out of the blocks and spurred on by a vocal local support assert their home field advantage early in games and see it gradually decline over the course of the match.This effect has been claimed in various other sports.However,there appears to be little or no evidence that this is the case in football.HFA can be measured in terms of the ratio of goals scored by the home side and if we take the 2010/11 season as being fairly typical we see that the ratio of home goals to away goals was identical to one decimal place last season in the EPL from the first half to the second..

Home Advantage by Half in the EPL 2010/11.

If we extend the study to 10 minute intervals at the expense of sample size,we find that the home/away scoring ratios remain fairly constant with the possible exception of the first 10 minutes.In every 10 minute period from 11 minutes onwards the percentage of home goals scored varies between a high of 61% and a low of 56%,not a huge variation considering we are working with just over 100 scoring events per period.However,the home side scores just 53% of the goals in the first 10 minute period.

Home Advantage per 10 Minute Period,EPL 2010/11.

This may or may not be significant.The first 10 minutes do tend to be slightly abnormal compared to the remaining 80+ minutes in other respects and the effect may be due to a natural caution at the outset displayed by both sides.Alternatively it may indicate that home teams are more passive in their initial approach or that away sides are able,temporarily at least to partly overcome the causes of a home team's natural home advantage.For the moment I'll just present the raw figures,but in later posts we'll see possible causes for a home team's advantage and how they can explain some of the variations we see in the historical data.There are likely to be many contributory factors,we've already seen here that away sides see red more often than they should and we'll also look at how team fare when they move to new stay tuned for "Home Advantage,Part 2 ."

1 comment:

  1. Very interesting.
    Hope you had happy Christmas.