Is the Home Ground Advantage Disappearing?

Home teams, as a whole, have not fared particularly well this season, which is one of the reasons that most of the MAFL Funds - all but one of which bet exclusively on home teams - have been testing Investors' patience.

Teams playing at home - truly or notionally - have won only 54.7% of contests so far this season, which puts them on track for the worst aggregate home team performance since 2001 when only 49.4% of home teams were successful. In the 25 seasons preceding this one, home teams have won at a rate lower than 55% on just two occasions: in 2001 as just mentioned, and way back in 1985.

To put this result in an ever broader historical context, here's a chart of home team winning percentages spanning the entire history of the competition.

Given that chart I think it's fair to say that Home teams have underperformed this year and that this is true no matter what historical perspective you choose to adopt.

(By the way, for the purposes of this chart and all those in the remainder of this blog, I've considered only those games that took place within the home-and-away portion of each season and I've treated as the home team that team that was designated as such by the AFL. I doubt that this latter treatment makes much difference to the general thrust of the analysis.)

Home ground advantage can, of course, be measured in terms other than whether the home team wins or loses. We can also look at the average points differential between the home and away teams.

Here's what we get if we do that:

From this we can see that teams playing at home have, on average, scored about 8 points more than their opponents (the actual average is 7.9 points per game), though this differential has fluctuated quite widely from season to season without ever dropping below zero for an entire season.

In this chart we see a similar trend in recent seasons to that which we saw in the previous chart. Indeed the pattern of the differential across the entire history of the VFL/AFL is quite similar to the pattern of home team win percentage, a fact borne out by the correlation of +0.69 that exists between the two sets of figures.

So far this season, home teams have averaged just 7.1 points more than their opponents, which, if continued for the rest of the season would produce the fifth lowest average of the past 25 seasons.

Closer inspection of the two charts above reveals that this season's poor showing by home teams is a continuation of a trend that started in 2006. The four full seasons and one partial season since then have produced five of the ten lowest home team percentage victories, and five of the ten lowest home team points differentials of the past 25 seasons.

Can we attribute this decline to a particular subset of teams?

Yes we can - collectively, the non-Melbourne based teams are to blame for the decline, as the following chart demonstrates.

Were it not for the astonishingly good home team records of the interstate teams in the early part of the decade, the aggregate performance of home teams would have slumped far earlier, dragged down by the diminishing success of Melbourne-based teams when playing at home.

Conversely, in more recent times, the overall decline in home team performance that we've witnessed would have been even worse had there not been something of a recovery in the rate of success of Melbourne teams playing at home.

The magnitude of these reversals in the fortunes of Melbourne and interstate teams at home is even more dramatic when we look instead at the home team points differential.

For the seasons 2002 through 2005, interstate teams playing at home scored about 4 goals more than their opponents. This year the difference has been less than 3 points.

In comparison, Melbourne-based teams playing at home in seasons 2002 through 2005 scored just 2 to 5 points more than their opponents - and actually almost 3.5 points less than their opponents across season 2003. This year they've averaged almost 10 points more than their opponents.

On balance I'd conclude that the advantage of playing at home is diminishing, which, if nothing else, will cause me to rethink the MAFL Funds for 2011 and beyond.