Opinion
Moneyball analytics originated in American baseball and was made famous in the 2011 film starring Brad Pitt.
Moneyball Pythagorean expectation uses mathematical formulae to estimate the number games a team should have won in a given season based on their for-and-against scores. The term comes from the resemblance to Pythagoras’ famous equation that we all learned in high school to calculate the length of sides of a right-angled triangle.
Comparing a team’s actual wins to their expected wins using this method shows which teams over-performed – teams that won more games than they should have – and conversely those that underperformed by winning fewer games than their scorelines indicated they should have.
In other sports, Pythagorean projections have been shown to be a reliable predictor of year-to-year improvement (or regression). Teams that over-perform in one season tend to slide in the following season with the opposite being true for under-performers as they revert to the norm. The predictive value of this increases with the discrepancy between the predicted and the actual number of wins, with at least one full game difference being generally considered the point of relevance.
A number of slightly different formulae have been developed, differing mainly with respect to the constants used to modify the formula for greater accuracy based on comparison to large sets of real-life data by statisticians. I am not aware of this having been done for NRL stats so I used the original Pythagorean expectation formula (method 1) and, for comparison, a modified version developed for American football, which I figured has similar scorelines to rugby league.
I do not have evidence, but I would expect method 2 to be more accurate for our purposes.
| Team | Predicted wins method 1 | Predicted wins method 2 | Actual wins |
|---|---|---|---|
| Storm | 20.5 | 19.6 | 20 |
| Roosters | 18.8 | 18.0 | 17 |
| Rabbitohs | 15.1 | 14.6 | 16 |
| Raiders | 16.6 | 15.9 | 15 |
| Eels | 13.7 | 13.4 | 14 |
| Sea Eagles | 13.5 | 13.3 | 14 |
| Sharks | 13.5 | 13.2 | 12 |
| Broncos | 10.3 | 10.5 | 11 |
| Tigers | 11.7 | 11.7 | 11 |
| Panthers | 10.0 | 10.4 | 11 |
| Knights | 11.0 | 11.1 | 10 |
| Bulldogs | 6.9 | 7.6 | 10 |
| Warriors | 8.1 | 8.7 | 9 |
| Cowboys | 8.2 | 8.7 | 9 |
| Dragons | 7.9 | 8.5 | 8 |
| Titans | 5.0 | 5.8 | 4 |
As can be seen, the majority of NRL teams in 2019 performed relatively close to their Moneyball prediction no matter which method was used.
Teams close to the one less win than expected – the under-performed – include the Roosters, Raiders, Sharks, Knights and Titans. These teams should improve.
If we assume method 2 is more accurate, the Titans, who had a 1.8 game difference, should rise. However even six wins would still see them on the bottom of the table.
The Rabbitohs and especially the Bulldogs appear to have over-performed and would be expected to regress in 2020.
Sorry, Bulldogs supporters, but your team won possibly three more games than they should have last year and are likely on the slide this season.
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