@AFLxScore is a Twitter Bot about to go into operation that will automatically post expected scores (xScore) of live scoring shots in the AFL mens competition. This will hopefully help footy followers get an idea of how skillful/lucky (or the opposite) each individual shot was. A team’s total xScore (vs actual score) gives an idea of seized or missed opportunities.
What is expected score?
xScore is a concept used in many sports (see xGoals, Expected Goals in soccer). It is a way of estimating what the average score a particular shot would score if it were taken over and over again, based on historical observations of similar attempts.
In soccer, it is a number between 0 and 1 as the most you can score with a single shot is 1 goal. In footy, the maximum score is 6.
In order to calculate an xScore, one needs to choose what factors they would consider in differentiating scoring opportunities. The most obvious, and possibly most important, is shot location: a shot from the goal square is expected to be much more successful than a shot on the boundary from 55m out. Other factors to consider could be:
- Shot context (set shot, free shot on the run, pressured shot),
- kick type (drop punt, snap, dribble kick, torpedo),
- the individual kicker,
- left/right foot,
- venue, and which end,
- weather conditions,
- if the player on the mark can move,
and so on. As there are so many variables that can be considered, a modeller has to draw a line somewhere. If too many factors are considered, individual sets of circumstances may have too few precedents to form a logical estimate (it could be skewed by a few lucky/unlucky shots). Additionally, some desirable factors may not have readily available data.
Hasn’t this been done before?
Yes, it has! The methodology I use is heavily influenced by Robert’s. I’m not doing anything particularly new here, I am just optimising this for live calculations using the best publicly available data, and automatically posting it to a Twitter account.
What data is available?
The data I have is near-live location data of all scoring shots, the player, the time, and the resulting score. From this I can determine the shot distance and angle (measured to the centreline). Save for manually recording additional data from the broadcast, that’s it. Post-game, when all the statistics are published, it’s possible to differentiate the shot context. However what’s important here is what’s available live.
As the data to be used is only distinguished by location, not context, the model built is fit using all available scoring-shot data with no shot context. This data is processed and a smoothed fit is produced to ensure all potential shot locations have a estimate consistent with historical scoring shots near it.
Hey, that doesn’t look right…
You’re right, it doesn’t! However, it’s important to realise that this is not saying that any shot taken from 70m+ out on a slight angle has a 30% chance of being a goal. The key limitation of the data I have is that it only consists of scoring shots. So what the above plot is showing is that if a score is recorded from 70m+ out on a slight angle, it historically has a 30% chance of being a goal.
A proper xScore measure would consider all attempted shots, including those that miss (out on the full, falling short, smothered, etc.). Any shot should have an xScore of between 0 (no score) and 6 (goal). In the methodology presented here when only scoring shots are considered, the xScore measures between 1 and 6.
What can the bot do?
After every scoring shot, the bot will tweet:
- the game hashtag, the game time and score recorded
- the player, distance, angle, expected score (goal accuracy %)
- the teams’ actual scores (and performance relative to xScore)
I may present this information in a different, more friendly way in the future as this is still a work in progress. I considered building it to send a pretty image of the shot trajectory with some data, but I’m aiming for this to be a simple messaging service to avoid heavily polluting people’s twitter streams with excessive images. Having said that, I may build some additional quarter-end reporting with some additional functionality as development continues.
Please follow if you are interested and any feedback to @AFLLab is appreciated!
Adam
AFL Lab 