An Analysis of Strength of Schedule for the 2019 AFL Season
/For quite a few years now I've used essentially the same methodology to analyse the AFL Fixture for the upcoming season (see this post from 2015, and, probably more relevantly, this one from the year before if you'd like the details).
This year, again I’ll use the MoSHBODS Team Rating System (rather than MoSSBODS) to provide the estimates of team ability and venue effects.
The 2019 AFL Fixture, released in early November (in its now customary impossible-to-import-directly-into-Excel format), has all 18 teams playing 22 of a possible 34 games, each missing 6 of the home and 6 of the away clashes that an all-plays-all full schedule would entail.
There is, again, a bye week for every team, these having been accommodated in 2019 by playing only 6 games in Rounds 12 through 14, which contrasts with the 2018 solution where we had successive rounds, from Round 10 to Round 14, with fixture lengths of 8, 9, 7, 6 and 6 games respectively.
In determining the 108 games to be excluded the League has, once again in the interests of what it calls "on-field equity", applied a 'weighted rule', which is a mechanism for reducing the average disparity in ability between opponents across the 198 home-and-away games, using the ladder positions of 2018 after the Finals series as the measure of that ability.
This year, of the contests that would pit a team from last year's Top 6 against a team from the Bottom 6, only 42 of the 72 (or about 58%) of the possible pairings are included in the schedule. That's the same number as we had in the 2018 Fixture.
By contrast, 46 of the 60 (or about 77%) of the possible pairings between the Top 6 teams are included, while 44 of the 72 (or about 72%) of the possible pairings between the Top 6 and the Middle 6 teams are included. That's two more and two fewer in those categories, respectively, than last year.
This year, 136 of the 198 contests (or about 69%) involve teams from the same one-third based on final official ladder positions last season. That’s four games more than we saw last year, the data for which appears at left.
The increases come from Top 6 v Top 6, and Middle 6 v Middle 6 clashes, of which we have two more in each category this season.
This year we'll again use MoSHBODS' opinions to to answer the following questions about the schedule:
How difficult is the schedule that each team faces, taking into account the teams faced and the venues at which they are faced?
How much has the use of the 'weighted rule' in truncating the draw helped or hindered a team's chances relative to a complete 34-round competition?
STRENGTH OF SCHEDULE
The first thing we need to determine to estimate a team's schedule strength is a measure of their opponents' underlying abilities. For this purpose we'll use MoSHBODS’ 2019 pre-Round 1 Team Ratings, which are set by taking 70% of their final 2018 Ratings, the regression towards zero reflecting the average historical shrinking in the spread of team abilities from the end of one season to the start of the next. These Ratings appear in the table below.
Again this year there are number of teams ranked quite differently by MoSHBODS than by their ladder finish.
Four teams are ranked more four or five spots differently by MoSHBODS compared to their official final ladder position, and seven more are ranked two spots differently.
Essendon, for example, will start the season as the 7th-highest rated MoSHBODS team despite finishing 11th on the final ladder, while Adelaide, ranked only 8th on MoSHBODS ratings, find themselves ranked 12th on the final ladder. Sydney are 12th under MoSHBODS but finished officially 7th on the ladder.
In the context of the AFL's competition "thirds", two teams would be placed in a different third were MoSHBODS to be used rather than the final ladder in defining the boundaries:
Geelong: Top 6 based on Ladder / Middle 6 based on MoSHBODS
Hawthorn: Middle 6 based on Ladder / Top 6 based on MoSHBODS
Last year, four teams would have moved into a different third using MoSHBODS rather than the ladder.
The average and range of the Combined MoSHBODS Ratings of teams from each of the AFL thirds is as follows:
Top 6: Ave +11.7 Points / Range 6.2 Points
Middle 6: Ave +3.9 Points / Range 10.8 Points
Bottom 6: Ave -15.6 Points / Range 19.3 Points
Ignoring Venue Effects, which we'll come to in a moment, the difference between playing an average Top 6 team and an average Bottom 6 team is therefore about 27 points (+11.7 less -15.6). That’s a couple of points larger than last season. Also, the spread of Ratings is - again and even more so than was the case last season - greater in the Bottom 6 than in either the Top 6 or the Middle 6. So, once again, it's more important exactly who you play from the Bottom 6 than who you play from the other 6s.
MoSHBODS also provides estimates of how much better or worse teams, on average, play at each venue. These estimates are known as Venue Performance Values, and are a logical extension of the notion of a "home ground advantage" to account for the fact that not all away venues are the same for a given team.
The current Venue Performance Values are summarised in the table below for all of the venues being used sometime during 2019. Note that teams need to have played a minimum number of games at a venue (four in the MoSHBODS System) before their Venue Performance Value is altered from zero (shown as dashes in the table below to improve readability).
Venue Performance values are, like Ratings, measured in Points, and are added to a team's underlying MoSHBODS Combined Rating when used in the Strength of Schedule calculation. So, for example, we can say that Geelong is, on average, a +2.7 Points better team than their underlying +9.6 Points Rating when playing at Docklands.
Because of the manner in which they are calculated, these Venue Performance Values incorporate the effects, if any, of interstate travel, which you can see, for example, if you run your eye along the row for the Gabba in the table above. At that ground, St Kilda are about a 14 point worse team, Hawthorn an 11 point worse team, and the Western Bulldogs a 10 point worse team. (Gold Coast are about a 19 point worse team at the Gabba, but you can’t really attribute that to the travel.)
After performing this calculation for all 22 games for every team, we arrive at the Strength of Schedule calculations below, within which larger positive values represent more difficult schedules.
In the left portion of the table we have the combined strength of the opponents faced by a team at home, split into the contribution from underlying ability and from the venue (from the opponents' perspective only). We would generally expect the Aggregate Opponent Venue Performance figure to be negative for a team, since their opponents are likely to perform less well away from home than at home.
We don't see that, however, for West Coast or Fremantle, because they are both playing their home games at Perth Stadium where no team but themselves has played enough games to register a non-zero Venue Performance Value. To the extent that travel is generally detrimental to teams' performances, this might overstate the effective strength of the opponents faced by these two teams.
(We also don’t see a negative value for GWS, but that is because a lot of interstate teams generally do relatively well when playing at the Sydney Showground.)
That said, even a notional negative 45 point allocation to Fremantle and West Coast - roughly the average figure for Adelaide, Port Adelaide, Gold Coast, Brisbane Lions, GWS and Sydney - would still have them with a Total Effective Strength of Schedule in the top 10 or 11 in terms of difficulty. Fremantle, we should note, is the only team from the bottom 6 on the 2018 ladder who only plays two of the four possible games against Gold Coast and Carlton.
Unadjusted though, based on their home fixtures, the teams with the five most difficult schedules (including venue effects) are:
North Melbourne (+47.8)
Richmond (+24.2)
Melbourne (+22.8)
West Coast (+9.9)
Fremantle (+5.4)
Such is the underlying strength of the teams they face, North Melbourne, Richmond and Melbourne would all still occupy the top three places even if we ignored venue effects. West Coast and Fremantle would fall to 11th and 12th, however.
Those with the easiest home schedules are:
Port Adelaide (-117.9)
Brisbane Lions (-63.0)
Hawthorn (-60.6)
St Kilda (-51.7)
Essendon (-46.4)
Were we to ignore venue effects, Port Adelaide, Hawthorn, St Kilda and Essendon would remain in the bottom five in terms of home schedule difficulty, while Brisbane Lions would actually move into the top five, such is the benefit to them of teams having to face them at the Gabba.
The middle section of the table looks at the combined strength of the teams played away from home, again split into the contribution from underlying ability and from the venue (from the opponents' perspective only). Here we would expect the Aggregate Opponent Venue Performance figures to be positive for a team, since their opponents are likely to perform better at home than their underlying ability would suggest. That is, indeed, the case.
Based on their away fixtures, the teams with the five most difficult schedules (including venue effects) are:
Hawthorn (+95.8)
GWS (+93.8)
Port Adelaide (+71.6)
Essendon (+71.0)
Collingwood (+60.3)
Ignoring venue effects would yield exactly the same top five in exactly the same order.
Those with the easiest away schedules are:
Brisbane Lions (-23.1)
Richmond (-12.0)
North Melbourne (-4.3)
Adelaide (-1.2)
Geelong (+14.7)
Ignoring venue effects would give the same bottom four, but drop Geelong into 6th and move Melbourne into 5th..
Combining the home and the away pictures to estimate a Total Effective Strength of Schedule (SoS) figure and summarising the results, we have:
Tough Schedules: GWS, Fremantle*, Collingwood, West Coast*, Melbourne, North Melbourne, Hawthorn
Slightly Harder Schedules: Essendon, Carlton, Sydney, Richmond
Average Schedules: Western Bulldogs, Gold Coast
Slightly Easier Schedules: St Kilda, Geelong
Easy Schedules: Adelaide, Port Adelaide, Brisbane Lions
* given zero venue performance values for all opponents at Perth Stadium. Fremantle would move to 'Slightly Harder' category with a -45 point adjustment, West Coast to 'Average'.
Comparing each team's ranking on Strength of Schedule with the ladder positions used for weighting the draw, a few teams stand out:
Fremantle and, to a lesser extent, Carlton have more difficult schedules than might be expected for a team in the Bottom 6
Richmond have an easier schedule than might be expected for a Top 6 team
Geelong, Adelaide and Port Adelaide have slightly easier schedules than might be expected for Middle 6 teams
To investigate the issue that some of these disparities might be attributed mainly to venue effects, I've included a couple of columns on the extreme right of the table, which calculate total Strength of Schedule using only the estimated underlying abilities of the opponents faced (ie the sum of the ratings of the teams played, ignoring venue effects).
Looked at that through this lens we see that GWS's and the Western Bulldogs' schedules appear considerably easier, while Sydney's and Port Adelaide's appear harder. No other team sees the ranking of the difficult of its schedule move by more than four places.
So, going back to the Total Effective SoS numbers, we find that the difference between the hardest and easiest schedules this year amounts to about 150 points across the season, which is a tick under 7 points per game, up two points compared to last year.
A 7-point advantage turns a game with an otherwise 50% victory probability into one with about a 58% probability, which converts to about 1.7 extra expected wins across a 22-game season. If, instead, we assume a 25% (or 75%) average probability without the advantage, then the 7-point advantage is worth about 1.3 extra wins a season.
If we exclude the teams with the two easiest and two hardest schedules the difference shrinks to about 4 points per game. That represents about 0.7 to 1.0 extra wins a season.
STRENGTH OF MISSING SCHEDULE
We can also view the schedule on the basis of the opportunities missed by a team as a consequence of playing only 22 of the possible 34 games.
The table below summarises the missing games in the 2019 Fixture, denoting with H's those games missed that would have been home games for a team. and as A's those that would have been away games. Note that I've ordered the teams on the basis of their final 2018 ladder positions, the same ordering that was used for implementing the AFL's 'weighted rule'.
West Coast, for example, fail to play two of the other Top 6 teams twice during the season, missing out on Richmond at home, and GWS away. Hawthorn misses Richmond and Melbourne at home, and Collingwood away. GWS misses West Coast and Melbourne at home, and Collingwood Away.
Ignoring venue effects, we can overlay MoSHBODS Ratings on this table to calculate a simplistic Strength of the Missed Schedule figure.
The column headed ‘Total’ shows the aggregate MoSHBODS Ratings of the opponents not played twice during the home-and-away season. The more negative it is, the weaker in aggregate are the teams not played twice; the more positive it is, the stronger in aggregate are the teams not played twice.
On this measure, Collingwood’s schedule was furthest away (in a detrimental sense) from what it would have enjoyed in an all-plays-all home-and-away fixture, Melbourne’s was second-furthest, and West Coast third-furthest. Conversely, St Kilda’s schedule was furthest away in a beneficial sense, Gold Coast second-furthest, and Carlton third-furthest.
As we'd expect, the magnitude of the number in that column for a team is broadly related to that team’s final ladder position, reflecting the AFL’s desire to have stronger teams play fewer games against weaker opponents and more games against similarly stronger opponents, and to have weaker teams play fewer games against stronger opponents and more games against similarly weaker opponents.
By adding back the effect on a team of not playing itself twice, we get a Net Impact of Missed Games figure, which is exactly equal to the negative of the Aggregate Opponent Ability Only column in the earlier Strength of Schedule Actual table.
CONCLUSION
Accounting for both the quality of the opposition faced and the venuing of fixtures (and using a venue performance value of zero at Perth Stadium for all teams except West Coast and Fremantle), I'd summarise the teams' relative Strength of Schedules as follows (with teams’ final ladder positions shown in brackets)
Tough Schedules: GWS (6th), Fremantle (14th), Collingwood (2nd), West Coast (1st), Melbourne (4th), North Melbourne (9th), Hawthorn (5th)
Slightly Harder Schedules: Essendon (11th), Carlton (18th), Sydney (7th), Richmond (3rd)
Average Schedules: Western Bulldogs (13th), Gold Coast (17th)
Slightly Easier Schedules: St Kilda (16th), Geelong (8th)
Easy Schedules: Adelaide (12th), Port Adelaide (10th), Brisbane Lions (15th).
Relative to the AFL’s intentions, you could make a case based on this listing that:
Fremantle, Carlton, Gold Coast and, maybe North Melbourne were hard done by (though you might have some methodological quibbles about the assessment of Fremantle’s schedule)
Adelaide, Port Adelaide, Geelong and Richmond did better than they might have expected (and you’d probably add West Coast to that list if you were quibbling about how Perth Stadium has been treated).
Were we to ignore venuing (which I don't think we should, but which I also recognise is harder for the AFL to account for), relative to those assessments:
Easier than shown above: GWS and Western Bulldogs
Harder than shown above: Sydney and Port Adelaide
Contributing to these differences is the fact that:
Interstate teams do relatively well at GWS’ main home ground, Sydney Showground
Interstate teams do relatively poorly at Sydney’s home ground, SCG
Interstate teams do relatively poorly at Port Adelaide’s home ground, Adelaide Oval
Strength of Schedule is quite a challenging metric to define and measure, and my final comment for readers is that I think you’ll get a better sense of it and be more able to make your own assessment by reading a variety of approaches.
Here are a few that I know about that you should check out:
