Thursday, January 24, 2019

The Wooden Finger Is Finally Getting Into the AFL Tipping Business

After almost a decade of my AFL Power Rankings, I’m caving in. (Or I’m expanding my horizons, if you want to look at it that way.)

I created my Power Rankings simply so I could get a better indication of each team’s strength than the ladder was showing me. I’ve never been that interested in using it to publically tip the results of matches. (Privately, I’ve done so for several years in various office football tipping competitions – not that I ever expect it to win the competition.)

But seemingly EVERY other ‘rankings system’ does tips. And it is interesting to see them. So yes, starting this year the Wooden Finger is getting into the AFL tipping ‘business’.

How does it work?

To get the predicted net margin of the home team:
To get the expected probability that the home teams wins, I am using a standard deviation of 36 points or six goals for the men’s league, and 18 points or three goals for the women’s league. Maybe it should be a bit more, but since I do all of my HGAs in terms of goals, and for my purposes it doesn’t make much difference, I’m going with those.

AFLW Round 1 predictions

Let’s start with my predictions for Round 1 of the new AFLW season, which is kind of nice in that there are less ranking systems that do AFLW tips.

Two notes of caution though, related to the entry of two new teams into the AFLW competition this year. First, the ranking points for the two new teams – Geelong and North Melbourne – are of course not based on any previous matches, but are sort of based on their premiership odds. Second, the entry of those teams has resulted in a fair amount of player movement, particularly from Collingwood to North Melbourne. It may take a couple of weeks for the ranking points to adjust to the ‘new’ strength of each playing group.

So there you go. My heart still lies in ‘explaining’ performance rather than predicting it. Nevertheless I hope these tips add another interesting and informative dimension to the rankings each week.

Tuesday, January 22, 2019

Which AFL Club Has The Easiest Fixture in 2019?

When the 2019 AFL fixture was released Champion Data tweeted this table, ranking each team’s fixture from hardest to easiest.

Champion Data rated Collingwood as having among the hardest fixtures, and St. Kilda as having among the easiest. So did Rohan Connolly. So did HPN Footy. So did FMI. Spoiler alert: I’m going to as well.

Note the shadings in the table above though. Those shadings indicate how Champion Data ranked the strength of each team – e.g. the ‘strongest’ teams are West Coast, Richmond, Melbourne, and Geelong. Champion Data copped some criticism in the comments under their tweet about those rankings. Oh boy, did some people get annoyed about that.

In particular, there was criticism that Geelong – who finished eighth in 2018 – was ranked in the top group, while runners-up Collingwood were not. Some people were incredulous that a team that was in the last finals spot and then eliminated in the first week could be ranked above another team that made the Grand Final. Yes, it may sound a little strange, but on this one my rankings agree with Champion Data.

Few would dispute that Collingwood had a better finals series than Geelong last year. Over the season as a whole though I’d say that Geelong was better, or at least they were similar. In the home and away season Geelong scored about as many points as Collingwood, and conceded about 150 points less. The Cats won two less matches, but had a much tougher fixture than the Magpies. Collingwood played lower teams Brisbane, Carlton, and Fremantle twice, while Geelong’s only ‘easy double-up’ was Gold Coast. Switch those fixtures around, and the Cats probably finish higher going into the finals.

One point of all this is that, while I don’t know exactly how Champion Data come up with fixture assessment, I expect it is very similar to how I do it. My ‘groupings’ would also be similar, with the exception that I would have Essendon rather than North Melbourne ranked in the top eight after its strong finish to 2018.

The other point is that, while the fixture won’t in itself make you a ‘good’ team, it can sometimes make a fair difference to where you finish on the ladder.

Rating each team’s fixture in 2019

My method for rating the fixtures is to add up the ranking points of all the team’s opponents over the season, while adjusting for home ground advantage. This means that the fixture rating is determined by:
  • which five teams the team plays twice, as well as that a team plays every team at least once except itself; and
  • net home ground advantage over the season.
As Champion Data’s table implies, it is the ‘double-ups’ that matter the most. I rate St. Kilda as having the easiest fixture this year, as it plays Carlton, Fremantle, and Gold Coast twice. On the other hand, I rate Collingwood as having the hardest ‘double-ups’, as it plays Melbourne, Richmond, and West Coast (and Essendon) twice.

Of course, it is likely that some teams will perform very differently in 2019 to 2018. In 2018 teams that played Adelaide and St. Kilda twice had easier fixtures than I initially expected, while teams that played Melbourne and North Melbourne twice had tougher fixtures.

St. Kilda’s ‘80-point’ advantage in the fixture compared with an average team isn’t by itself going to make up their -500 point differential from last year. If the Saints significantly improve though it may well help them with a finals push. It probably helped Richmond to a better finals spot when they improved in 2017, and as mentioned above it probably helped Collingwood last year.

St. Kilda finally wins out in the fixture

I’m in no way a Saints fan, but they were definitely overdue for some ‘love’ from the fixture. They had one of the worst fixtures last year, and the year before that, and in 2015 and 2014 as well.

St. Kilda’s tough luck in the fixture in recent years was well covered in Squiggle’s article ‘How The Fixture Screwed St. Kilda’. In recent years the Saints have had tougher ‘double-ups’ than a team that finished in their position typically would. Also while net home ground advantage is usually fairly minor, it has mattered more in St. Kilda’s case. The Saints have tended to get less home matches against non-Victorian opponents compared with the amount of times they have had to travel across the border.

Again, the fixture by itself didn’t make St. Kilda miss the finals last year, and it likely won’t get them in the finals this year if they don’t otherwise improve. It’s good for them though, after recent years, to have a fixture that is a bit kinder to their chances.

Wednesday, January 16, 2019

Hello Quinn Wheatley

AFL WOMEN’S Power Rankings 2019: Ranking AFLW’s Two Newest Clubs

In a month’s time the AFL Women’s (AFLW) competition will kick off its third season. At the same time I will start up the third season of my weekly AFLW ‘Power Rankings’. These rankings aim to give a more accurate reflection of the ‘current’ strength of each team than the ladder does. They adjust each team’s results for the strength of its opponents, as well as whether they played the match home or away.

For my rankings though the start of the third AFLW season brings with it a new challenge. Two new teams will be joining the competition – the Geelong Cats and the North Melbourne Tasmanian Kangaroos. (Next year will be even more challenging.) How should I rank these two new clubs?

Initial ranking points: look at the premiership odds

I faced this type of situation before when I was trying to work out how to initially rank the eight clubs in the AFLW’s first year. Back then I decided the teams should not be rated as equal, since the ‘consensus’ view – as reflected in bookmakers’ premiership odds – was that they were quite different in their ability.

This also looks to be the case this year for Geelong and North Melbourne. North Melbourne are rated as a very strong side, and have the second-lowest odds to win the premiership, after last year’s premiers the Western Bulldogs. Meanwhile Geelong are rated as a relatively weak side, with the longest odds to win the flag.

Therefore I am going to start North Melbourne off on a relatively high rating of about +5 points, which is what the teams that were initially the highest ranked started on in 2017. Conversely I will start Geelong on a relatively low rating of about -7 points, which is the same as the lowest ranked team at the beginning of 2017. (There is a bit more behind my decision, but I don’t think the full method is either interesting or ‘scientific’ enough to go into all of the specifics here.) All of the teams’ ranking points are also adjusted slightly, to keep the average number of ranking points across teams at zero.

This will start North Melbourne off in fourth spot on the rankings, and Geelong in second last. The rankings update pretty quickly though because of the small number of matches, with 25 per cent of a team’s ranking determined by its most recent match alone. Hence a much better indication of the strength of these teams should emerge within a couple of rounds of the new season, as it did in 2017.

Home ground advantage: think GWS

As there is only a small history of AFLW matches to draw from, I base my adjustments for home ground advantage (HGA) on the adjustments I use for the men’s league (see table below). While I haven’t done a big review of my men’s league HGA adjustments since those rankings began in 2010, they are still based on far more data than we have for the women’s league to date.

In the men’s league I give Geelong the same HGAs as NSW teams against Victorian clubs whenever the Cats are playing in Geelong. Therefore, my HGA for the Cats in Geelong will be based on that of GWS. Following this rule and using my HGAs from last year would give Geelong an HGA of four points when playing against Victorian or NSW teams in Geelong, and eight points when playing against other clubs.

What to do for North Melbourne, which will play its home games in Tasmania? In the men’s league my HGAs for Hawthorn and North Melbourne when playing in Tasmania are smaller than their HGAs when playing in Victoria, as they play less often there. That doesn’t seem right for a genuine ‘Tassie Kangaroos’ team though. Alternatively, I could give the Roos the same HGAs as the other ‘Melbourne clubs’ – that is HGAs against non-Victorian teams at home, but no HGAs against other Victorian teams.

However I’m going to take the view that, since Tasmania is their permanent ‘home turf’, North’s HGA against other Victorian teams in Tasmania should at least be as much as the Cats in Geelong. After all, Victorian teams have to travel further to Tassie than they do to Geelong. On the other hand, I don’t think I should make them any higher than when Victorian teams travel to Sydney. Let’s make North Melbourne’s HGAs the same as Geelong’s and GWS’ then.

Note though that in the men’s league I assume Geelong is not disadvantaged against Victorian teams when they play at other Victorian venues. In AFL, Geelong plays a fair amount of games at both the MCG and Docklands. In AFLW though that looks like it will not apply. Therefore I will also add a disadvantage for the Cats (and Roos) when they play elsewhere in Victoria.

One other thing – in general, I’m not going to use the same HGAs as I have the first two years. When I first came up with my women’s league HGA adjustments I didn’t know what the volume of scoring would be. Given the scoring we have seen so far, and since I am already making some adjustments to last year’s rankings because of the new clubs, I am going to take this opportunity to reduce them slightly.

In matches where GWS, Geelong, and North Melbourne play the ‘Melbourne clubs’ I’m going to reduce the HGA from four points to three points. In all other matches involving clubs from different cities I’ll reduce the HGA from eight points to six points.

These new adjustments do not make much difference to the ranking points, but seem more in line with the scoring we’ve seen over the first two AFLW seasons (indeed, they may still be a touch high). The table below lists the new HGAs, and compares them to the men’s league.

Revised rankings: Magpies now on top, but they are not the same team in 2019

As mentioned above, these revisions make little difference to the rankings (see table below). Collingwood regain the top spot it lost to the Brisbane Lions after the Grand Final. The Lions’ showing in the GF against the Dogs in Melbourne is now considered slightly less impressive with the reduced HGA.

While the Pies’ top ranking may seem peculiar given they only finished mid-table in 2018, it is because they finished the year off really well. They had strong wins against Melbourne and Adelaide, an away win against Brisbane, and a close loss against the Bulldogs. Given that Collingwood is in the weaker half in the premiership odds, maybe one should be thinking about the Magpies as a ‘good bet’ then?

Not so fast … what the premiership odds likely reflect that the rankings do not is that Collingwood lost some top players to North Melbourne, including Jess Duffin, Jasmine Garner, Moana Hope, and Emma King. The Magpies have also lost AFLW Rising Star winner Chloe  Molloy for the season due to injury. Top-end talent matters more in AFLW than the men’s league, and the forward prowess that we saw from Collingwood in the second half of 2018 is likely to be significantly curtailed this season.

Indeed you could make an argument for a more significant shake-up of the rankings given the off-season player movements. As I said above though, if these player movements do result in large changes in performance the system will correct quite quickly. For the most part the revised rankings are not too far off what ‘the market’ is saying about how teams are expected to perform this season, so I’ll leave the tinkering there.