I’ve been excited in the past couple of years to have been watching the growing use of analytics in sports. In particular, as a fan, I’ve been following what’s been going on in hockey (recommended: mc79hockey and Eric T.) as well as soccer (Statsbomb). My goal as a rugby fan, player and coach, is to start digging into what the oval ball game has to uncover.
So, with that in mind, time to start work on some of the basics. Inspired by what’s been done in examining the value of goal differential, I thought I’d begin by examining point differential in rugby union.
Quick primer for the uninitiated; rugby has four ways of scoring points:
- Scoring a try (5 points)
- Converting a try (2 points)
- Kicking a penalty goal (3 points)
- Kicking a drop goal (3 points)
The team that is able to score more points in 80 minutes wins the game. Simple.
The question, then, for those interested, is what does it matter if a team wins a game by one point or ten? A win is a win, and surely that is all that matters, no?
Using the common Bonus Point System in use in Super Rugby, Pro 12, and the English Premiership (I’ll tackle the French Top 14 in a separate post), teams earn points in the standings as follows:
- 4 points for a win
- 2 points for a draw
- 1 “bonus” point for scoring 4 tries (or more)
- 1 “bonus” point for losing by 7 points (or fewer)
- No team may earn more than 5 points in a match
Looking at the relationship between points scored for and against a team is actually a pretty good indicator of the success that the team will likely have in the standings. As shown below, in three leagues, over ten seasons, there is an incredibly strong correlation (R^2 = 0.92) between a team’s average points differential per game, and their ability to collect points in the standings (NB: R^2=1.00 would be a perfect relationship).
Despite the fact that there are many differences in style, tactics, etc between the southern hemisphere Super Rugby and the northern hemisphere Pro 12 and Premiership, that relationship between point differential and points in the standings largely holds over the long run (for reference, I use the following seasons: 2005 to 2014 Super Rugby; 2004/05 to 2013/14 Premiership; and 2004/05 to 2013/14 Celtic League/Pro12).
However, there are some differences between the two hemispheres with regards to the value of each point.
In each league the average point differential will be zero (Total Points For = Total Points Against). An average team in Super Rugby will score more points (and allow more) than in Pro 12 or the Premiership. Based on 10 seasons of history in Super Rugby, we can see that this average team would collect 2.63 points per game played in the standings (in a 16 game season this works out to 42 points). If they scored one more point per game on average than their opponents, this would increase to 2.73 points per game (or 43.7 points). One point more per game in Super Rugby results in nearly 2 points gained in the standings in a season.
In Pro 12 or the Premiership (which have similar scoring rates), an average team (0 points differential) would collect 2.38 points per game played in the standings (in 16 games: 38 points). If they scored one more point per game on average than their opponents this would increase to 2.49 points per game (or 39.8 points).
While the math is slightly different, the overall impression is the same. Now obviously there will be teams that both over and under perform with regards to their performance, but this is how important point differential is to a team’s chances in the standings. For each additional point in differential scored per game, a team could gain around .10 to .11 in standings points per game. In a 16 game season (like Super Rugby) this could result in 1.7 additional points in the standings. In the Premiership or Pro12, where a season is 22 games long (but scoring lower), this addition could result in a bump of around 2.4 points in the standings.
That is how important one point can be. And ten? Well, if that same average team in Super Rugby added ten points in differential per game, they would likely collect 3.64 points per game in the standings (or about 58 points overall). This year, the top team in Super Rugby standings were the Waratahs. They had 58 points. That is what can be the difference between winning by one point or ten.