threenewpadlocks
Brownlow Medallist
West coast get on average 8 more free kicks per game then their opponents. This is the biggest reason their record on the road isn't as good as at home.
Why - from a scientific perspective - home ground advantage actually exists is one of the most fascinating things that you can look at, if you're the type of person who likes to spend their free time learning and investigating why things are.How can you say that with such confidence? That's an empirical question, and would require a close review of their games. Other factors, including player mentality and ground shape may play as big if not a greater role in their home vs away game record.
Officiating is no doubt the greatest single factor above all others individually - though me separating "travel" (ie the physical impact of time zones and physically having to be on a plane) and "home conditions" (the familiarity with the conditions) as two separate things, when it's kinda hard from a scientific perspective to separate the two as one comes with the other.
There's a book that came out this year, Footballistics, that basically looked at the two types of free kicks where play style doesn't have an impact (ie a team that tackles more or is first to the ball or whatever)- running too far, and deliberate rushed behinds/out of bounds - and found that the vast majority of these are paid toward the home team. Once you divide this by the impact that this has on the margin, they found that home-biased umpiring constitutes something like 30-40% - 3 to 4 points out of about an average 10 point home ground advantage the AFL has. Worth noting that this is the average of all games - no doubt some umpires are better than others, and it's not like 3 points happens every game, sometimes it'll be a goal or more impact, and sometimes none at all.
The most famous of these studies came in German soccer, where roughly half the grounds have an athletics track and the other half doesn't. They found that when controlling for the size of the crowd, there's a stronger home ground advantage in the grounds where there's no athletics track, as the crowd is closer to the ground and the noise is louder.
Another study looked at Ice Hockey players and their testosterone levels on home and away ice, and found it was higher at home - the primal nature of protecting your location helped their performance.
I'm inclined to think officiating has a lesser effect as a % of total home ground effect than all other sports - for a couple of reasons. One, we have three umpires. Two, we have different sized grounds, so the home advantage has an impact more than other sports with familiarity to the ground size - think Geelong at Kardinia playing the narrow ground more effectively. Three - because the game is more physical, requiring greater running and hits and the emphasis on recovery is greater than almost all other professional sports - the very nature of having to get on a plane and be in the air for a long time, is more significant than it is for other sports.
But all factors considered, officiating is still the main factor, even if it's less than 50%. The others combine to around 60-70% but in smaller factors in combination.