Tuesday, 19 January 2016

The Elusiveness of Home Field Advantage.

A month ago I suggested that the widely supposed demise of home field advantage in the Premier League was overstated and the near equivalence of home and away wins in the first 17 match days wasn't widely different from a number of sequences of 170 matches seen in the recent past.

Home teams had also been unusually frequently charged with red cards compared to recent history.

Although I wasn't predicting an immediate switchback, home sides have fared extremely well in the ensuing month, winning 25 of the 40 matches. Overall for the season home teams now have won an unexceptional 40% of games compared to 31% for the visitors.

We may have simply witnessed simple variation within a relatively small sample of matches and the betting markets certainly continued to factor in a degree of home advantage. But a contributing factor in the rush to declare home advantage a thing of the past may have been the choice made to express the perceived effect.

Comparing the percentage of home wins against away victories or points per game for host and visitor appears a fairly uncontroversial selection. However, in extreme cases the choice may be a poor one.

If we consider two equally matched teams playing a season's worth of games against each other where home field advantage is a typical 4 tenths of a goal. In this case the effect of home field advantage is well illustrated by taking the percentage of home wins (on average 46% in 10,000 sims) and compared it to away wins (~26%).

Home teams also score an average of 0.4 goals more than away teams.

Still No Place Like Home.
If however, we make one side greatly superior to the other and simulate a season again with a home field advantage baked into the simulation, it becomes much less obvious that a home field advantage exists by simply looking at win percentages for the home and away teams.

When we change the competitive balance of the matches between two teams to mimic the type of extremes we might see between the best and worst in the Premier League "home field advantage" as measured by win% nearly "vanishes".

Home teams win only a few % of games more than away teams achieve.

The much better team in this artificial simulation wins a high proportion of home games and a similarly high proportion of away matches bringing the home and away win % close together.

It is only when we look at goal differential do we see that the home teams are scoring on average nearly half a goal a game more than the away team over the simulated season, despite a near equivalence of home and away wins.

These are extremes, but if a batch of games bears a greater resemblance to the second simulated scenario than to the first and respective home and away win percentages are quoted rather than say goal difference between host and visitor, we may erroneously conclude that home field advantage has disappeared. When it has merely chosen not revealed itself by a measure that may be sensitive to the competitive balance of the sample of games played.

No comments:

Post a Comment