Tuesday, March 4, 2014

Equality in the AFL Over Time

Over at The Footy Maths Institute recently, there was a guest post by Sean Ross about equalisation in the AFL. Sean argued that equalisation is the most important issue facing the AFL, and that without it the AFL risks having a wider gap between the ‘haves’ and the ‘have nots’. For example, the teams that Sean classified as ‘haves’ occupied six of the top seven on the ladder in 2013.

That got me to thinking: how equal has the AFL been – just in terms of results, not finances or memberships or whatever – over time? To determine this I used the Noll-Scully measure of competitive balance. (Don’t worry Melbourne fans, the measure has nothing to do with Tom Scully.) For a particular league, this measure compares the actual standard deviation of team’s winning percentages to the idealized standard deviation, where the idealized deviation depends upon the number of home-and-away/regular season games played. The lower the Noll-Scully ratio, the more equal results have been – for example, the National Basketball Association in the US historically has a high ratio, while the more even National Football League historically has a low ratio.

The graph above shows the Noll-Scully ratio in the AFL for each season. As you can see, the ratio jumps around from season to season, so the bold line, which is a moving average of this ratio, smooths out some of this volatility.

As Ross Booth has previously noted, the AFL became more even after 1985, with the introduction of the salary cap and then the draft (Figure 1, p. 65 in his article is essentially the same as my graph above). Backing up Sean’s argument is that inequality in results has ticked up in the past few years. However, most people would concede that is at least in part due to the introduction of historically bad new teams Gold Coast and Greater Western Sydney (there was also an uptick when new strugglers Footscray, Hawthorn, and North Melbourne entered the VFL in 1925).

Another part of Sean’s argument though is that there has either developed or there is a risk of developing a stronger positive association between a club’s financial/crowd-pulling power and its ladder position. The graph above doesn’t show whether that is true or not – it shows the level of inequality in results, but not what is causing it – but I guess we’ll see whether the big clubs, as they did in 2013, continue to make up most of the finals teams in the years ahead.

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