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Why history and form will see Souths beat Penrith despite the odds

Mark McGrath new author
Roar Rookie
30th September, 2021
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Mark McGrath new author
Roar Rookie
30th September, 2021
59
1454 Reads

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.

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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 of the Panthers is tackled

Isaah Yeo (Photo by Matt King/Getty Images)

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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

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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:

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  • 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?

South Sydney Rabbitohs fans

(Photo by Cameron Spencer/Getty Images)

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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.

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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.

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