Here's some more NRL mathemagic from http://www.matterofstats.com/mafl-s...afl-and-the-nrl
Win % = 1 / ( 1 + (Pa/Pf)^1.89 )
where Pa = Points Against, Pf = Points For.
Derived from something called Pythagorean Expectation, you can determine the winning percentage for a team over a season only taking into account their Points For-Against. The linked blog attests that the same formula can be used to determine the % chance of victory in a match, given the relationship between the scoring abilities of Team A and Team B. If a team is expected to score 5% more points than their opponent, you can plug it into the formula to determine the probability of their victory (54.7%).
So taking that into account, we can work out what the line should be:
On Centrebet, Cronulla is $1.50, Gold Coast is $2.65. Presuming that the overround is
evenly split (which it likely isn't; the home team is probably of greater likelihood of winning than the odds indicate), according to the bookmaker at least, Cronulla has a 63.85% change of winning.
If we plug that into our formula, we eventually get the relationship between the two as:
Points for Cronulla = 1.351356 * Points for Gold Coast
Now what? Well, the bookmaker also tells us there's a 50% chance of the total score being above 37.5. Using our ratio and the total volunteered by the bookmaker, we can solve simultaneous equations and discover that to comprise the 37.5 total, Gold Coast = 15.95 and Cronulla = 21.55. The margin between the two is 5.6. In 50%
of the thousands of imaginary games these two sides can play (the 50% with points total greater than that specified), to maintain the points ratio this will be the smallest possible margin. So if you can find Cronulla -5.5 at greater than $2
, the bookmaker is telling you you're going to win!
Guess what price Cronulla -5.5 is? $1.91, of course.
The bookmaker's line is the same as determined by the formula above (and the odds are lower than the total offered implies) for all games this round bar Sydney Roosters vs Canberra Raiders. In this case the calculated minimum margin is 17.4. Roosters -16.5 is currently paying $2.05 (Woohoo! Roosters -17.5 is greater still: $2.10).