Across many sports, there is a distinct advantage to being the home team.

Looking up the stats for some leagues across the world, we can see a consistent theme:

• In basketball, NBA teams win 62.7% of their home games, and home ground advantage has been shown to be associated with a 15% increase in winning percentage.
• International cricket is similar, with 60% of wins going to the home team.
• Rugby follows this trend, with the home team winning 58% of the time.
• AFL stats show that the home team wins 58.3% of the time.

OK, so home teams have an advantage in pretty much every sport. What's the big deal with the AFL then? Why do people keep complaining about "Vic Bias"? Well, the problem with the AFL is that the grand final is locked to the MCG. Other (more professional and unbiased) sporting codes will generally give home finals to the higher ranked/seeded team, or rotate the location regularly - so everyone gets a shot at having the home ground advantage. Across a large enough number of seasons, everything will average out. The AFL, though, has locked in the grand final to the MCG until 2059. This means that Victorian clubs get a default "home ground advantage" against interstate sides in every single grand final.

At this point, we've established two facts:

1. Home ground advantage exists in the AFL.
2. Victorian teams are sometimes given an unfair/unearned home ground advantage in grand finals vs interstate sides.

But there's one last question to be asked - has this ever actually unfairly changed the result of a grand final?

I'm going to investigate every year going back to 2000 in order to try and answer the above question. There are two components to this question: Firstly: Was there any Vic Bias? Secondly: Did the Vic Bias impact the outcome?

To determine whether there was any Vic Bias, we consider the following scenarios:

1. Victorian side vs. Victorian side: No Vic Bias.
2. Interstate side vs Interstate size: No Vic Bias.
3. Victorian side vs. Interstate side, but the Victorian side "deserved" a home final by virtue of finishing higher in the H&A season: No Vic Bias.
4. Victorian side vs. Interstate side, and the Interstate side "deserved" a home final by virtue of finishing higher in the H&A season: Vic Bias, as the Victorian team was given an unfair home ground advantage.

Scenarios 1-3 don't have any Vic Bias (or, at least, none due to the MCG lock-in). Scenario 4 does, though. But to actually determine whether this impacted the result, we need to consider how big the home ground advantage is. For example, if we can determine that home ground advantage is worth 2 goals, and the Victorian team wins by 10 goals, then it's clear that the home ground advantage didn't change the results. Conversely, if we've determined that home ground advantage is worth 2 goals, and the Victorian team wins by 2 points, then it's reasonable to conclude that they may well have lost the game had they not been gifted an unfair/unearned home ground advantage. For the purpose of this analysis, I'm going to calculate the home ground differential for each team by subtracting their average away game margin from their average home game margin. I then take the midpoint between the two grand finalists home ground differentials as the overall home ground diffential. If this value is less than the game's actual margin, then I'm going to conclude that the Vic Bias did not impact the final result. If the home ground advantage was MORE than the game's margin, then I'm going to conclude that the Vic Bias impacted the final result. Without further ado, here are the results:

Very interesting results! Almost one third of the grand finals going back to 2000 have been impacted by Vic Bias, but - luckily - none of them have actually had their result changed by the Vic Bias. It will be interesting to see how these results play out over the next 36 years (or beyond).