Tuesday, 23 July 2013

Home Specialists Are Largely Random.

One of the universally accepted truths about professional team sports is the existence of a home field advantage or the ability of sides to produce consistently better results in the familiar comforts of their own stadium compared to those on their travels. The causes of hfa are likely to be many and varied and a few have been examined in this blog. The sport where the scope to match a causative agent to the observed effect is in the NFL, where pitch dimension and uniform regulations are one of the few constants on any given Sunday (or Monday or Thursday...)

Artificial turf dwellers, as well as domed teams can draft track athletes as wide receivers and quarterbacks with booming arms to take full advantage of their benign home conditions, but may find the harsher realities of a mid December trip to Lambeau Field, Wisconsin less productive. Natural advantages, such as the thinning, stamina sapping, mile high air in Denver has traditionally made the Broncos a harder nut to crack at home compared to when they are the visitors, once the usual six point swing from switching venues is allowed for. Since 1989, Denver has enjoyed hfa of around 4.5 points, compared to a league average of 2.8 over the same period.

Green Bay, the tenants of Lambeau, have, since 2009 done even better than the Broncos and posted a hfa of over 5 points a game in an era where the average league figure is in gradual decline. The stadium at Lambeau may never host a Super Bowl, but their harsh climate can guarantee a better than average home performance. Or maybe not.

If we instead start counting in 1989, when presumably Wisconsin winters were just as harsh and hfa across the years was 2.8 points, the Packers won their home matches by an average of 7.2 points and their road trips by 1.2 points. So over the long haul the Pack's hfa is around 3.2 points. Still better than league average, but well short of the near unconverted touchdown seen in recent years.

So the question now becomes, is Green Bay's recent, elevated hfa just simply a random, small sample fluctuation from the long term league mean to which their results from 1989 onwards have more closely adhered to or can their fans risk frost bite along with their sport, safe in the knowledge that the harsh weather is disproportionately helping their side and confounding their opponents. If it's the latter, it is only for eight matches a year of sometimes sub zero football, discounting the playoffs.

Simulating an NFL game isn't easy, they really should have awarded one point for a touchdown and scrapped the rest of the scoring events, so for the remainder of this hfa post I'll return to football or soccer, to make the distinction clearer.

Expressing a side's talent differential compared to their opponents can usefully be done in terms of the average number of goals they might concede and score in a contest. A fairly typical match between two equally matched sides would see the hosts scoring around 1.5 goals and the visitors about 1. From here it is a short, but tedious step to simulate all manner of match outcomes.The final score and more usefully the margin of victory or defeat is most helpful in determining the range of hfa that might be seen over a simulated 38 match EPL season once random chance intervenes in matches with a perfectly normal hfa.

In the plot above, I've simulated 10,000 38 game seasons for an average Premiership side and I've assumed that they enjoy a typical home field advantage of 4 tenths of a goal in each home game they played compared to each of their 19 away matches. HFA is typically expressed as comparisons between success rates (games won + games drew/2 as a percentage of total matches played) at home and on the road or by venue specific goal differences. I've chosen to use goal difference and have plotted seasonal averages from the simulation grouped in tenths of a goal per game over the 38 game campaign.

The most common average HFA value seen over a season in the simulation is, not surprisingly centered around 4 tenths of a goal. But the variation in recorded values for a side with the usual level of HFA baked into the figures is perhaps surprising. A signification minority of the time, a 0.4 HFA side can over the short course of 19 home and 19 away matches record a "negative" HFA or in other words they produce better results as visitors than they do as hosts.

The plot also contains similarly rare, but significant occasions where their home results are vastly superior to their away ones. These particular seasons are nailed on to be described as home specialists, yet the outcomes are merely the relatively unlikely product of a normal HFA season being replayed enough times for the right hand tail of the plot to show itself.

Finding real life dopplegangers of these rare, but fully expected  deviations from a side's true HFA isn't difficult. Twelve seasons ago, West Ham won 75% of their total points at home, compared to a league average of just over 60% and slightly further back in May 1998 Crystal Palace bowed out of the Premiership having collected just 33% of their meagre total of points at Selhurst Park. And every year numerous sides delight their travelling fans, but then regularly frustrate their "stay at home" majority the following weekend.

Neither WHU nor Palace continued the trends of their anomalous seasons in the subsequent campaign. West Ham fell to 53% of points gained at home and Palace went on a neat WHU like home spree in gaining over 70%. As we added more seasons, the long term average HFA for each side trended towards more usual levels

A team can record levels of performance that appears far removed from their underlying talent in small sample sizes and in this regard, HFA is no different to other documented stats. Regression towards the mean, rather than external forces is invariably the greatest influence on season to season HFA correlation and if you look often enough, a team with an underlying normal HFA is going to turn in a "home specialist" looking 38 game performance.

It is unlikely to stand the test of time.

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