The 2018 6 Nations is underway and all of my predictions for week 1 turned out to be correct (just!) with the help of an amazing drop goal drop goal by Johnny Sexton. This means that out of 37 matches I have used rankings to make predictions, 30 have been correct.
World Rugby’s goal is that rankings can be used as a predictive tool. The idea is to take each team’s current ranking and add 3 points to teams playing at home. Whoever has the higher ranking is then expected to win but the closer the rankings, the closer the game is expected to be. The current World Rugby rankings as of 7th February 2018 is shown to the right along with their ranking points and the change (if any) from the previous week.
Last week, I broke down my previous predictions into 3 groups as below and the updated numbers are:-
- Where the ranking gap was less than 3 points – Out of 11 matches, 7 were correctly called.
- Where the ranking gap was between 3 & 6 points – Out of 9 matches, 6 were correctly called.
- Where the ranking gap was 7 points or more – out of 17 matches, all were correctly called.
These figures are shown by the black line in the next chart. What I did here was build a logistic regression model to estimate the probability that the prediction would be correct given a certain ranking gap and this shown by the green bars (shaded differently for each of the 3 groups above). Such a model allows me to add an additional feature to my predictions, the chances of my predictions being correct.
You can see these probabilities in the next table which lists my predictions for the 2018 6 Nations along with actual results. As of now, it is only week 2 that can be counted as a true prediction with the other weeks treated as indicative for now. This is because World Rugby will update the ranking points after each week which of course will lead to different predictions.
Looking back at week 1, Wales v Scotland was supposed to be a coin toss but much to the disappointment of the Scottish fans, Scotland did not turn up and Wales had an easy romp in the end. Ireland were supposed to dispatch France without too much trouble but in the end, they needed an extraordinary drop goal in the 82nd minute to win it. Only England performed as expected.
For week 2, my predictions are for 3 home wins without too much fuss. Note the ranking gap in all 3 games exceeds 7 points and so far I am 17 out of 17 for such matches. There is always a first time though and as an English fan, I just hope it is not at Twickenham!
That aside, these predictions are pointing to an England v Ireland grand slam decider in the last week. Of course we now have bonus points as well and whilst I have no grounds for predicting bonus points, I have constructed a provisional league table with the following system. Where the ranking gap is less than 7 points, the expected losing team gets a losing bonus point and where the ranking gap is 14 points or more, the winning team gets a 4 try bonus point. This results in the following table. Whilst this shows England winning a Grand Slam, if you multiply England winning probabilities for the 5 games, you find their probability of winning a Grand Slam is only 50%. Personally I would surprised if there was a Grand Slam in 2018.