There are only two important factors in determining the outcome of this Sunday’s NRL grand final.
The difference between the Penrith Panthers and the South Sydney Rabbitohs are the week off and their recent form.
The huge advantage of the week off
There have been nine grand finals since the McIntyre finals system was dumped by the NRL after 2011. Since then, only one team, North Queensland in 2015, have managed to win the premiership without having a week off. Three others made the grand final without a week off and failed: Manly (2013), Canterbury (2014) and North Queensland (2017).
That’s a success rate of just 11.1 per cent. Yet the average probability of these no-week-off contenders winning the grand final (based betting markets) was 36.3 per cent. This means teams without the week off have a success rate 3.2 times less than they should.
But even this 11.1 per cent success rate is overblown, because it doesn’t take into account all the teams that didn’t make the grand final without the week off.
Since 2012 there have been 36 teams with no week off that have attempted to win the premiership. Only four of those made the grand final and only one of those won it. So that’s one team out of 36 – a 2.8 per cent success rate.
(Photo by Ian Hitchcock/Getty Images)
The converse of all of this is that the week-off teams have a 97.2 per cent success rate in grand finals since 2012 and have won 88.9 per cent of the time when they have played a no-week-off team.
Looking at the points scored in grand finals also shows the advantage of the week off. Since 2012 the average score in grand finals between teams that have had the week off compared to those that haven’t is 26 to 12.
But you could say that the teams in these grand finals with the benefit of the week off were expected to win anyway, so that statistic doesn’t mean that much.
Okay. Let’s now measure the performance of these teams against how much they were expected to win by.
If we use the line betting market as the barometer for how much each team was expected to win or lose the grand final by and then calculate how much above or below expectation each team performed, then on average the team with the week off performs 8.5 points above expectation.
So even with the betting markets factoring in the week-off advantage for these teams, they still outperform the market by 8.5 pts on average.
But you could say that all of this has nothing to with the Penrith team of today – different teams, different opposition and even a different game with the new rules.
Isaah Yeo (Photo by Matt King/Getty Images)
Souths superior recent form
Let’s use recent form to find how good each of the grand finalists are. There are two models that I think have merit here: power ratings and the DAT model.
Power ratings
Power ratings is a system of rating teams that produce the lowest margin of error between actual and predicted margins of victory (MV) for all of the match results. They use the points scored for each team and a calculated home-ground advantage for each venue. This is what I have used here. The power ratings produce an MV prediction error lower than the betting markets.
Now, you could use the results from the whole season, but historical analysis shows that using only the results for the post-Origin period is a better predictor of finals success. Doing that and producing power ratings on these results (from Round 18 and later) produces the following for the grand finalists.
Penrith Panthers: 7.9 power rating points
South Sydney Rabbitohs: 25.8 power rating points
Expected home margin of victory: -17.9
DAT Model
DAT stands for defence attack territory. It’s a model I developed to rate teams based on their on-field performance rather than the points they score. It is based on the following performance variables:
- kick return metres (KRM);
- line breaks conceded (LBC); and
- net kick metres (NKM), which is team kick metres less opposition kick metres
These three team variables were identified as the most strongly correlated variables with margin of victory (MV) when analysing the 2014-20 seasons. The DAT model ratings produce a slightly lower MV prediction error than the betting markets for 2021.
When applying this model to the results from Round 18 for this season we get the following ratings for the grand finalists:
Penrith Panthers: 12.3 DAT rating points
South Sydney Rabbitohs: 19.7 DAT rating points
Expected home margin of victory: -7.3
Both of these models based on performances from Round 18 onwards find that Penrith are rated considerably lower than Souths. These findings also accord with the historical analysis of grand finalists through the prism of whether they have the eweek off or not.
But the betting markets have this around the other way, with Penrith favoured to win by 1.5 points. So what is going on here? Do the betting markets know something this analysis doesn’t?
(Photo by Cameron Spencer/Getty Images)
Why the betting markets are betting the other way
No, not really. There are two related things going on here.
Firstly, there is a strong media-driven narrative around Penrith missing out last year and being desperate to make amends this year. This narrative has been justified by Penrith being one of the two best teams for most of the season. This has fuelled betting demand for Penrith, which leads me to the second point.
As a top-ranked performer for most of the season, bookmakers have taken a large amount of money on Penrith to win the premiership. In contrast, Souths, who struggled in the early to mid part of the season with 50-plus-point floggings from Melbourne and Penrith, not surprisingly haven’t been backed much to win the premiership.
The end result is that once it became a Penrith versus Souths grand final, bookmakers were overcommitted on Penrith and undercommitted on Souths. So to balance their books and to ensure a profit no matter who wins they have to offer an attractive price on Souths.
These two reasons are why Souths are currently $2.25 and Penrith are $1.67 to win the grand final.
History and recent performances are strongly on the side of Souths here. Media fandom and needles will not save Penrith from the accumulated fatigue over three slugfests in consecutive weeks.
Souths, with the confidence of already having a finals win over Penrith, a team loaded with representative players, and a belief instilled from a master coach, means that history will indeed repeat for those that enjoy a week off and have the best form running into the finals.
Rabbit Stew
Guest
Yet... They Lost! I think I’ll have another look at that Dubbo Game. Most enjoyable!
Mark McGrath
Roar Rookie
Without going into detail SM, I have found that over the last 10 years, a power rating of form post Origin is a better predictor of GF performance than a whole of season power rating. To me this makes sense. Teams can be very good going into Origin, falter on the way out and underperform in the finals series. Conversely, team can underperform in the first half of the season, then improve in the 2nd half in the run into the finals after Origin. Both of these scenarios would support a post Origin form analysis being superior to a whole of season approach. It is very hard for a team to be consistently very good throughout the whole season. The trick is to be peaking at the right time. I think Wayne Bennett knows this and it's why he has Souths playing their best football of the season at the moment.
Short Memory
Roar Rookie
Mark, great article and I love your use of the stats. I do wonder though whether a reliable statistical analysis of predicted performance in the GF should exclude Origin affected games? The loss / work overload of key players is typically disruptive of the top ranked teams, which could skew stats in favour of teams with less players affected by Origin. Any thoughts?
Big Mig
Roar Rookie
“It is only prudent never to place complete confidence in that by which we have even once been deceived.” ― René Descartes
Short Memory
Roar Rookie
Gee BM, I can't say I've seen much in the way of outrage and bias from any Panthers supporters on here. That mostly seems to be coming from Bunnies diehards.
Big Daddy
Roar Rookie
Mark it's quite a good article if your into stats and they are very meaningful but as you said people's preference is purely driven by emotion as you you well can observe . At the end of the day the only thing people will remember is the score after 80 minutes on Sunday . I haven't got a horse in the race it's only gut instinct . TBH both teams have pro's and cons and it wouldn't come as any surprise if either team wins.
Glory Bound
Roar Rookie
And to that end BD wouldn't it be nice for once for the team that wins the premiership to win fairly and not che@t their way to a premiership? Just for old times' sake? :stoked:
Forty Twenty
Roar Rookie
Those odds are very deceptive O.S . The best defensive team has had a 50% strike rate of winning the title but how about the 2nd , third and 4th best defensive outfit? None of those are going to be anywhere near 50%. If you go by those figures I'd guess (wildly) the Bunnies would be closer to 5 or 10% over time. 10 years is a very skinny time frame anyway. I've always claimed the refs have a big say in who wins plenty of games and this could be no different.
Opposed Session
Roar Rookie
Mark, thanks for that but I understand how these effect game trends. I meant the formula for NKM and how that offsets the outcome of your prediction. I realise you probably can’t share that publicly. I’m Always trying to get an advantage over my opposition and find hidden details to coach to better outcomes. For your players it’s about proving to them why it’s important, if they understand it they are more likely to understand it’s importance to the outcome.
Opposed Session
Roar Rookie
FT, FYI only 5/10 last NRL premierships have won by the best defensive team in season. The refereeing will have a huge influence on the outcome of this GF. The last 2 weeks of finals have not been refereed the same as in season. If it’s allowed to become a wrestle like Penrith’s games v Parra and Storm then that definitely suits one team more than the other. If it’s like games in season then it suits both teams. Souths can win right handed but not left handed I reckon. Penrith can win either hand. They say in boxing that “styles wins fights” unlike boxing, the referee influence in RL is far greater. So whatever style he allows to play will determine what kind of contest we see tomorrow.
Forty Twenty
Roar Rookie
A pretty strong stat is how the best defensive team typically wins the comp but can the Pennies fire up again? No mention of the 50 point curse? Does the young age of the Pennies help them counter the fatigue factor? What's happened to the ''boring and predictable'' two horse race thing? The stats are firmly favoring a team which isn't allegedly isn't even in the race. The Bunnies win the comp , some of the finals games are brutal , low scoring upsets , Manly make the 4 , the Titans make the 8 ahead of the Raiders, Turbo wins the M after missing much of the season , the Chooks are making up the numbers for most of the season and the only horse in the race come finals time , the Storm, are in the stables when it matters. Can't complain about that. I'm still tipping the Panthers because I claim their resolve will be as strong as any team who have played in a GF and they have incredible defence. If the Bunnies defend like they did against Manly , they are gooooorne but if the play like they did against the Pennies a couple of weeks back they possibly run away with it. I think that was their peak though. Latrell Mitchell hasn't been missed but he will be.
Mark McGrath
Roar Rookie
LBC indicates defence — Line breaks, not surprisingly are highly correlated with tries. So, teams with a low average LBC will have good defence. One of the truisms of rugby league is that defence wins matches, more so in the big games where the quality of the teams are higher. KRM indicates attack and energy — Teams with strong KRM’s usually produce strong metres gained in their sets. This then usually translates into more attacking opportunities and in turn tries. KRM also governs a team’s energy level. Teams with a strong KRM means that forwards will have less ground to cover returning upfield after an opposition clearing kick. This will then translate into more energy for attack and defence particularly in the back end of each half as fatigue starts to set-in. NKM represents territory — When I first discovered that net kick metres was strongly correlated with margin of victory I was a bit perplexed. What does how long you kick have to do with how far you win or lose by? The answer is that NKM is a proxy for territory. Teams that dominate territory do less long clearing kicks and more short kicks when they are in the opposition’s 20m zone. These teams will tend to produce a negative NKM value. Conversely, teams that struggle to gain and hold territory have a positive NKM value because they will have to kick long more of the time and short less of the time.
Tony
Roar Guru
:laughing:
Opposed Session
Roar Rookie
Mark, I’d love to see the breakdown on how you calculate the DAT model. Love the article, stats are subjective which is probably why people on here are abrasive in their response. I have long held the KRM and a couple of other stats in high regard as they go a long way to determining the top 4. It reflects down through competitions also the old NYC and both state comps all impacts this way. LBC makes total sense. NKM is why I want to understand your take on this. As you know, many variables can affect NKM for either side.
Mycall
Roar Rookie
Other than Penrith and Storm, who would you consider to be good opposition? With the exception of Parra V Storm, no team outside the top 3 provided decent opposition to the top 3 sides.
Mycall
Roar Rookie
Computer says "No"
The Barry
Roar Guru
Haha…agreed Saying what’s happened before is meaningless is pretty strange. Whatever method we use to tip or back teams relies on what’s happened earlier. The alternative is that every game is literally a coin flip Sorry if my earlier comment sounded narkier than I meant…that 97% number just jumped out at me. Really did enjoy the article and hopefully look forward to more (next season). FWIW I tipped Souths too.. but not with a huge amount of confidence…
The Barry
Roar Guru
Monosyllabicly…
The Barry
Roar Guru
Just reflecting on the article… LBC and NKM make immediate sense but do you think KRM is just the flip side of NKM. typically a team that is kicking off the front foot kicks deeper, finds the grass more regularly, pins the back three down deeper. A team kicking off the back foot makes less metres, usually hits the fullback on the chest, they get easier metres So a complicated way of me asking is this sort of measuring the same thing twice…
Ado Potato
Roar Rookie
Yeah, what Michael Brogan just said. Go Souths!