Friday, July 12, 2013

Mathematically possible - GWS making the AFL finals

(Update: thanks to Gazza White and the AFL subreddit for linking my post - it's already by far my most popular blog post!)

Towards the end of a sporting season, it's not unusual to hear the commentators call a team a "mathematical" chance to achieve some target - be that winning a premiership, making the finals, avoiding relegation, whatever. What this means is that there is at least one combination of events (usually discounting other teams being disqualified) that could bring it about, but it's almost vanishingly unlikely to occur.

Very seldom is this a more appropriate term than for the current chances of Greater Western Sydney getting into the top 8 and making the AFL finals this year - so much so that commentators probably aren't even aware that it is a mathematical possibility.

Here is the current AFL ladder as of the end of Round 15 (courtesy of FanFooty - note that the official AFL ladder is not actually up to date!)

Team P W D L For Agt Percent. Pts
Hawthorn 14 12 0 2 1645 1167 141 48
Geelong 14 12 0 2 1556 1216 128 48
Essendon 14 11 0 3 1483 1142 129.9 44
Sydney 14 10 1 3 1379 1048 131.6 42
Fremantle 14 10 1 3 1201 954 125.9 42
Richmond 14 9 0 5 1387 1190 116.6 36
Collingwood 14 9 0 5 1321 1225 107.8 36
Pt Adelaide 14 8 0 6 1317 1158 113.7 32
West Coast 14 7 0 7 1404 1277 109.9 28
North Melb. 14 6 0 8 1435 1210 118.6 24
Carlton 14 6 0 8 1331 1219 109.2 24
Adelaide 14 6 0 8 1288 1228 104.9 24
Gold Coast 14 5 0 9 1197 1341 89.26 20
Brisbane 14 5 0 9 1133 1451 78.08 20
W. Bulldogs 14 4 0 10 1102 1433 76.9 16
St Kilda 14 3 0 11 1129 1337 84.44 12
Melbourne 14 2 0 12 981 1775 55.26 8
W. Sydney 14 0 0 14 1003 1921 52.21 0

In green is our team of interest - Greater Western Sydney. They are currently winless at the bottom of the ladder, 8 wins behind the lowest top 8 side (Port Adelaide - in red). Unfortunately for GWS, there are also 8 games left in the season, so one thing is immediately clear: GWS must win all 8 of their games, and Port Adelaide lose all 8 of theirs, for GWS to be any chance of making the finals (the two teams do not play each other, so this accounts for 16 separate games). If this happens, the ladder looks like this:

Hawthorn 52
Geelong 52
Fremantle 46
Essendon 44
Sydney 42
Richmond 36
Collingwood 36
Port Adelaide 32
GWS 32
West Coast 28
Carlton 28
Adelaide 28
North Melbourne 24
Gold Coast 24
Brisbane 24
Western Bulldogs 16
St Kilda 16
Melbourne 8

This on its own is still not enough to guarantee GWS a place, however - there are 9 other teams on the ladder that are also striving for a spot in the top 8. For GWS to make the finals, none of these sides can finish with more than 32 points (8 wins) at the end of the season. Therefore every game that involves one of these sides - 46 games, excluding the 16 already accounted for by GWS and Port's games - can make or break GWS's finals chances. In particular, West Coast, Carlton and Adelaide cannot get any more than 1 win for the rest of their remaining games. In fact, there are only 10 games that don't affect GWS's chances - the games between top 7 sides, who already have more wins than GWS can possibly get and are guaranteed to place above them on the ladder.

Using a computer to calculate the possible combinations in which this could happen comes up with 150,744 ways for GWS to place equal 8th. Even assuming that all teams will have a 50-50 chance of winning each game for the rest of the season (discounting draws), an assumption which is very kind to GWS to say the least, this would give them a 150,744 / 262 = 3.27 in a hundred thousand billion chance of finishing equal 8th on points.

To put this into perspective, imagine a lottery where you have to pick which 6 balls out of 40 will be drawn - a 1 in 3.8 million chance. Now imagine only entering that lottery twice in your life - and winning both times. Even THAT would be twice as likely as GWS finishing equal 8th, on a good day.

Notice that I've mentioned GWS finishing equal 8th. Even this herculean feat doesn't guarantee them a place - in the very best case scenario of the 150,744, there will be 6 teams vying for 8th place on 32 points (on average in these scenarios, there will be 9.6). So GWS's best-case scenario looks like this:


Geelong 76
Fremantle 66
Hawthorn 64
Essendon 60
Sydney 54
Collingwood 52
Richmond 48
Port Adelaide 32
Carlton 32
Adelaide 32
Gold Coast 32
Western Bulldogs 32
GWS 32
West Coast 28
North Melbourne 28
Brisbane 28
St Kilda 28
Melbourne 28

To make the finals, from this point they need to gain a higher percentage than the other 5 teams. Currently, they are on 52.21%, having scored only 1003 against their opponents' 1921 points. On the other hand, their currently best-placed opposition, Port Adelaide, has a percentage of 113.7%, scoring 1317 to their opponents' 1158.

This informative site tells us that the average score in an AFL this season so far is 92.43, and the average margin for a game is 36.92. So a roughly "average" game of AFL would involve the winner with 110.89 points and the loser with 73.97. If we assume that GWS's 8 winning games follow this scoreline, as well as Port's 8 losing games, then we end up with GWS having an improved percentage of 75.22% and Port with a dented percentage, but still plenty enough for finals, of 93.33%.

So, obviously just winning is not going to be enough for GWS to leapfrog Port and its other finals rivals. Let's assume the same as above, but this time work on the assumption that GWS has somehow found a secret scoring weapon and is able to rack up ridiculous scores while keeping their opponents to an average score of 73.97. They would need to be able to score, on average, 167.78 points in order to beat Port's percentage - an average winning margin of 93.8 on their run home - and hope that none of their other rivals have had a similar late-season percentage boost themselves. I'll leave it to someone else to work out how often a team has won 8 games in a row by an average margin of at least 93.8 in AFL history.

Our conclusion: is it possible for GWS to make the finals? Mathematically, yes. Are they going to make the finals? No. But it'd be a hell of a story if they did!


  1. That's a fantastic read!

    You did forget one crucial element - and let's face it is far more likely than the Giants making the finals - is the remote possibility of a team losing points.

    Let's say for instance a team -- I'll call them Shmessendon - have been involved in a performance enhancing drug scandal. And they have all there points stripped away as a result. Making West Coast's 7 wins, and not Port Adelaide's 8, the target. How does that change things?

    Not being mean! But would me still buying two lottery tickets be better odds?

  2. Unfortunately for GWS, a lot of the issue is in the other bottom 10 sides - all of those 150,744 combinations still involve a lot of other teams on 8 wins, even taking Port's 8 wins out of the picture. So even with West Coast on 7 to beat, GWS *still can't afford to lose a game* because the fixture is set up such that some of the closer bottom 10 teams WILL win games (draws may make that interesting, however...)

    Though the other teams' percentages are generally worse than Port, note that North Melbourne's percentage is actually *better*, and it just so happens that in the scenario I chose they weren't one of the finals competitors. So the chances are still going to be WAAAAY below two lottery tickets odds. The percentage alone is probably an unprecedented hurdle even if they got the necessary wins/losses going their way.

    Though who knows - maybe the drug scandal and GWS's current predicament is all part of Sheedy's grand plan, and he's the puppetmaster pulling all the strings? ;) Maybe the derby will be the beginning of the greatest run to the finals ever seen? (it won't)