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Footy Forensics: Is the AFL the world's most even sports league?

This AFL season is on pace to be the most even of any major sports league, and among the most even in footy history.

If you think there has been a lot of close games so far this season, you're not imagining things. There have been 126 games of footy in the 2017 season so far, and 22 of those have been decided by 6 points or less, including the weekend's thrilling draw between the Cats and Giants. That's 17.4% of games decided by a goal or less - the highest proportion of close games in the first 15 rounds of a season in AFL history. At this point last year, barely 10% of games had been decided by a goal or less, and that was an increase on 2015.

DATA: AFLTABLES.COM.

Not only has the season been stuffed full of close games, but it's been a crazily unpredictable year full of upsets, too. Not many tipped the faded Hawks to beat the Crows in Adelaide a couple of weeks back, nor the Lions to top Essendon at Docklands, but those are just the latest big upsets the season has thrown up. Nothing makes any sense, and the universe is a random and unpredictable place that will make a mockery of all attempts to impose order on it. It's been a tough season for footy tippers, is what I'm saying.

In 37.6% of games so far this year, the team that was favoured by the betting markets has lost. That's the equal highest rate of upsets in the AFL in the period since 2009, for which we have betting market data. If we consult our Elo model, there have been more upsets this year than in any previous AFL season.

DATA: AFLTABLES.COM. HISTORICAL ODDS FROM AUSSPORTSBETTING.COM.

How even is the league overall? A common way to measure the evenness of a sporting league is to look at the standard deviation of teams' win percentages. You remember standard deviations from high school maths - that slightly annoying formula to calculate how dispersed something is. If the standard deviation of win percentages is high, that means that the competition is uneven, while a low reading means that the competition is closely balanced.

The 2017 season is the second most even in AFL/VFL history, if we compare each season after 14 games played. The 1997 season was the most even ever at this point in the year, but the current season isn't far behind. It's quite a dramatic turnaround from a few years ago. In the late 2000s, the league was fairly uneven, and then the introduction of the Suns and Giants to the competition keep things unequal for a while. But now, the pendulum seems to have swung back dramatically towards competitive balance in the league.

DATA: AFLTABLES.COM. HISTORICAL ODDS FROM AUSSPORTSBETTING.COM

Last week for ESPN, Nick Riewoldt said he thinks the AFL might be the "most tightly and evenly contested professional sports league in the world." It turns out the numbers back him up - the AFL, at least in the 2017 season so far - is the most evenly balanced major sporting league in the world.

We've compared the competitive balance of sporting leagues using the standard deviation of win percentage, but adjusted using a standard formula to account for the fact that teams in different leagues play a different number of games. The standard deviation in each league is compared to what it would be if each game were decided by a coin toss. This year, the AFL's competitive balance is very close to what you'd expect if each game were decided by the flip of a coin.

Of the leagues we looked at, the NBA is the least balanced. The standard deviation of basketball teams' win percentages in 2016-17 was about two and a half times the level you'd expect if each game had been decided by a coin toss. Last year's AFL men's season was also quite imbalanced as far as international sports go, right up around a similar level as the English Premier League and other major soccer leagues. The AFLW competition was relatively even in its short 2017 season, but the 2017 AFL men's comp has leapfrogged it to the top of the competitive balance leaderboard.

DATA: LEAGUE WEBSITES, WIKIPEDIA. IDEAL STANDARD DEVIATION IS 0.5/sqrt(N), WHERE N IS THE NUMBER OF GAMES PLAYED BY A TEAM IN THE LEAGUE.

The AFL does a lot to try and ensure parity in the league. The salary cap and draft system does a lot to even things up, and the league gives poor teams a further leg up through the fixture. The AFL would have to be happy with the results this year - upsets abound, there are close games aplenty, and for now at least the league is the most evenly balanced in the world.

Matt CowgillMatt Cowgill is an economist by day and footy stats obsessive by night who thinks there's no such thing as too many graphs. He uses data analysis to tell stories for ESPN's Footy Forensics, as well as his own blog The Arc.

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