6 Nations 2018 – Who will win

After two weeks of the 2018 6 Nations, all 6 of my match predictions have been correct. Whilst this might be vindication of the World Rugby ranking model, the last thing we want in sport is a perfect prediction model otherwise what’s the point? Where would be the excitement of watching sport live? So I am hoping for an incorrect prediction soon …

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.

So far, the success rate is 33 correct predictions out of 40 matches involving 6 Nations teams in the 2017 & 2018 6 Nations and the 2017 Autumn Internationals. These can be broken down as follows:-

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 20 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 3 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 2, Ireland V Italy was the blowout that was expected with Ireland getting a bonus point. The other two matches were also won by the home team but given that both Wales & France got losing bonus points, this suggests hard fought matches closer than indicated by the win probabilities.

For week 3, my predictions are wins for France, Ireland and England. France’s game is apparently the surest bet but given the recent turmoil in the French team and the attacking endeavours of the Italians, this might be closer than at first sight. The other two games have ranking gaps of less than 7 points and given my record is 2 in 3 for such matches, I think an upset is on the cards. Again I hope it is not England!

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.