Primer – What is a Pythagorean expectation?

Pythagorean expectation is the idea that you can calculate a team’s winning percentage based solely on its for and against. It originated with baseball nerds but, according to its Wikipedia article, has been adapted for other sports. It is also where the name of this site came from. Pythago is basically what Pythagoras would have been if he had been Australian.

To rip straight from Wikipedia –

“The basic formula is:

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Primer – What are Elo ratings?

Elo ratings originated in chess as a way to rate different players. A player starts with a rating of 1500 by convention and then the rating goes up or down depending on whether the players wins or loses. The player’s rating will change proportionally to the rating of their opponent: if the player beats a very highly rated opponent, the player’s rating will go up by more than if they beat a minnow.

This rating system has been adapted for soccer, NFL, AFL and a few systems have been developed for rugby league teams (1, 2 & there was a third but it seems to have gone missing). What intrigued me was how The Arc and 538.com were able to take Elo ratings and, instead of just ranking teams, were able to predict the outcome of matches in advance with a surprising degree of accuracy. I decided that I wanted to try the same thing for the NRL.

Here’s the soccer explanation for how it’s calculated:

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NRL Projections Update – Round 4, 2017

Note: post published on April 5 but backdated to suit the date it should have been posted.

Round 4 Results

sydney city-sm Sydney 20 (4-0) d souths-sm South Sydney 6 (2-2)

penrith-sm Penrith 40 (2-2) d newcastle-sm Newcastle 0 (1-3)

brisbane-sm Brisbane 13 (2-2) d canberra-sm Canberra 12 (1-3)

manly-sm Manly 36 (2-2) d canterbury-sm Canterbury 0 (1-3)

cronulla-sm Cronulla 20 (2-2) d parramatta-sm Parramatta 6 (2-2)

north qld-sm North Queensland 32 (3-1) d gold coast titans-sm Gold Coast 26 (1-3)

melbourne-sm Melbourne 22 (4-0) d wests tigers-sm Wests Tigers 14 (1-3)

st george illawarra-sm St George Illawarra 26 (3-1) d warriors-sm New Zealand 12 (1-3)

[Home team in italics]

Projected Wins

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NRL Projections Update – Round 3, 2017

Note: post published on April 5 but backdated to suit the date it should have been posted.

Round 3 Results

melbourne-sm Melbourne 14 (3-0) d brisbane-sm Brisbane 12 (1-2)

canterbury-sm Canterbury 24 (1-2) d warriors-sm New Zealand 12 (1-2)

gold coast titans-sm Gold Coast 26 (1-2) d parramatta-sm Parramatta 14 (2-1)

souths-sm South Sydney 24 (2-1) d newcastle-sm Newcastle 18 (1-2)

sydney city-sm Sydney 14 (3-0) d penrith-sm Penrith 12 (1-2)

manly-sm Manly 30 (1-2) d north qld-sm North Queensland 8 (2-1)

canberra-sm Canberra 46 (1-2) d wests tigers-sm Wests Tigers 6 (1-2)

st george illawarra-sm St George Illawarra 16 (2-1) d cronulla-sm Cronulla 10 (1-2)

[Home team in italics]

Projected Wins

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