Expected Goals are all the rage today and it is certainly a great addition to the toolbox we use in analyzing and understanding football. It gives an understanding to the match beyond just the ultimate results, goals. It values long term-performance over short-term results, since long-term performance will eventually return as results.
It is also something that we do this all the time in real life using more simplistic statistics, like averaging numbers. Having said that, there is a right and wrong way in using Expected Goals as a tool.
Briefly speaking, here is how I understand Expected Goals to work. I apologize if I am rehashing something that the readers already understand, but it is important to my thought process to write these things down. Since I am of average intellect and capability here is how I see Expected Goals.
A – stuff/shots team A does.
B – stuff/shots team B does.
C – stuff the referee does (refereeing decisions influence games).
D – the fluky stuff/shots (bad bounces, 40-yard screamers, deflected goals).
E – Goals.
By looking at the above variables, we can easily see the relationship in every match is:
A + B + C + D = Goals!
What Expected Goals endeavour to do is to take out some of the D, the fluky stuff. The reasoning behind this is that fluky stuff is not sustainable in the long term, and therefore they are assigned a lower value accordingly. That 40-yard screamer that was scored – it is revalued at 0.02 (probably even less) instead of the 1 goal it ended up being recorded as in the results.
So in the long term, all things being equal, we cannot reliably take 40 yard shots and expect to affect the score. Perfectly encapsulating that long-term performance is more important. It basically tells the audience and managers the quality of the shots.
Now assuming that Expected Goals encapsulates all the fluky stuff for shots (it doesn’t, but in terms of shots, we can assume as such), the equation becomes:
A + B + C = Expected Goals.
What Expected Goals does not do, is account for refereeing errors. We know this well because if Expected Goals is a judge of performance and the game can be affected by the referee, it stands to reason that Expected Goals are affected by refereeing decisions. For example if referees give loads of freekicks from good positions and they are all directly taken as shots, the Expected Goal value will be “expected goal value from that position” x loads. So referees definitely affect Expected Goals.
The next step, as with every statistic, is how you implement it into your understanding of the situation. I have previously written on Untold about how paid journalist should do a better job on using statistics. In the previous article, which in hindsight, I poorly titled “Intentional misrepresentation, misunderstanding of statistics, or…?” I tried to impress upon the fact that I was not judging the player himself (Lacazette), rather my criticisms are of the journalist who used statistics that were neither convincing nor particularly rigorous.
In the article titled “Arsene Wenger deflects as Manchester City roll on” an ESPN writer slags off Wenger for criticizing the referee and using it as a deflection tactic. In particular he uses Expected Goals as the justification that Manchester City were a better team despite the refereeing errors.
One irksome sentence was, “Or perhaps he wanted us to ignore the fact that, despite flurries of better play from Arsenal, this was still a hugely one-sided game, as the 1.83 to 0.31 expected goals count shows.” I would implore the reader to read his article to make sure I am not misrepresenting him. Once again, I emphasize that I am not here to say whether his criticism of Wenger is right or wrong, rather the statistics used are not appropriate.
In this particular game, there was an offside goal, and according to many, a questionable penalty. Both are directly influenced by refereeing decisions, whether they are right or wrong. What the writer is doing in effect is ignoring refereeing decisions. Remember that offside goal? From that position, a shot carries an Expected Goal value of about 0.7 or 0.8, and that penalty, also carries an Expected Goal value of about 0.3 or 0.4 (assuming it counted as a big chance rather than penalty, if counted as penalty, this value is almost doubled).
So subtracting 1.8 from 0.8 and 0.4, we get 0.6. Thus in terms of Expected Goals, we get 0.6 vs 0.3, Man City vs Arsenal. Based on the performance of both teams on the day, this is the value of goals I would expect basically, anywhere between a 1-0 or 0-0; flip of the coin stuff.
Lastly, I would point out that even without the Expected Goals statistic, we know one thing from experience; Goals Change games!!!! A bad refereeing decision that results in a goal can change the performance and therefore the Expected Goals of a team.
Statsbomb has a particularly good article on this. So the ESPN writer not only misuses the Expected Goals number, he also disregards that erroneous goals can change games. Non-disclaimer: I do not work for Statsbomb I just really enjoy the articles since they are discussed statistically well, even though I do occasionally disagree with their interpretations.
In conclusion, we cannot determine the game performance of teams without considering refereeing influences with Expected Goals, so maybe there can be a statistic for that in the future. If we use Expected Goals and expect referees to have no part in it, it is like saying that the food tastes bad but the ingredients do not matter.
In short, paid journalists should write better articles. And I apologize if I showed ignorance in any of the analysis; please let me know if there are any statistical things I misunderstood, I am amateur and not paid so at least I have that excuse.
Good supporting Arsenal!
- Who will Arsenal’s next manager be and what we can learn from Moyes?
- How the evolving referee scandal in Germany is causing the English media problems
- How fake news was used to manipulate the Man C story to avoid considering the issues