Plugging the round 3 results into the model gives us the following ratings:
Not so much change this round, apart from the Australia’s surge into 2nd position. Australia are the clear form team of the RWC so far – their rating has increased 6 points since round 1. Interestingly, their home advantage score has declined, meaning that they are becoming are more formidable unit away from home (note that some of their headline rating increase is a result of this – ratings points shifting from the home advantage to the general rating as they have improved their away results). Ireland have also dropped 3 points following their narrow win over Italy.
Finally, NZ have not changed much at all, even despite being heavily criticised for their performance against Georgia. In fact, the result against Georgia was not too bad – the model picked a 41 point margin, whereas the final margin was 33 points (almost a “Green” result, actually). The so called minnows are not as bad as people expected – this is borne out in the results as well as the ratings (i.e. the general result that “minnow” games are closer than previous RWCs should not really have come as a surprise).
One interesting result of the model is that it can give us some indication of the home & away performance of the teams compared to their overall rating, and also (perhaps) of each team’s form. Looking at the prediction errors on the model’s training set, we can see that the ratings for some teams appear to over- or under-rate either their home or away performance somewhat. This is partially a result of the way the model fits ratings to results – the squared error means that big errors are penalised proportionally more than small ones, and the model may forgo symmetry in order to avoid big errors, even if this means that it gets some games slightly more wrong (the idea being that one big error is worse than several small ones).
This is very indicative only – it is really not much more than a intuitive interpretation of the model residuals (errors) and has nothing very scientific or statistical behind it. None-the-less it is interesting I think. Examining the results from the training set (which included all RWC games thus far), we see that most teams are rated fine for both home and away. However, for some teams the ratings may be skewed slightly. England for example, may be under-rated away – that is, they perform away slightly better (win by a little more or lose by a little less) than they should given their rating over the 2014-2015 period. Australia does the opposite.
Note that, as the RWC games are double weighted, some of what we see here in the Away column (except for England) may result from this, with good wins in recent games providing the skew. That is, the away column could be interpreted as an indicator of recent form, with “Over” indicating that the round 3 rating is better than the earlier 2014-2015 results suggest, and “Under” the opposite. In that case, Australia and to a lesser extent Scotland are the form teams and Ireland is somewhat out of form.
Don’t put much emphasis on this – as I said this has very little solid behind it – but it can provide at least some more useful information when interpreting the model results and predictions.
And so to the final round predictions:
This round we have a model first – a draw prediction in the first game between Canada and Romania. The model really can’t decide between these two teams, so a draw it is…
Otherwise, in key match-ups deciding the QFs South Africa to comfortably beat the USA, Scotland to deal with Samoa, Australia to beat Wales (just), and Ireland to beat France (I disagree with the model on this one, but we’ll see). And England to go out on a win against Uruguay.