Analytic insights and dubious corners stats

Does Manchester City’s much publicised analytic insight on corners stands up to scrutiny?
There are reasons to doubt it!

Corners are one of the key moments in a match that grab fans’ attention.  There is always high expectation that a corner will result in a goal. However, the probability that a corner will lead directly (first touch) to a goal is very low [1]. Investigating the stats suggests that more goals come from the penalty area scramble that often follows corners, and any goal that is scored tends to come many touches of the ball after the original corner kick.

However, it is difficult to prove or disprove these hypotheses either way, in view of the lack of trustworthy data publicly available.  Manchester City appear to have tried (they can afford to buy or collect the data), and their claim on corners has received much publicity as a key finding of their massive analytic’s effort (11+ analytics people). Shown below are some extracts of how this claim has been reported in the media.

City had gone 22 games and not scored a goal from a corner. After this….. they scored 8 goals in 15 games.”
(http://thevideoanalyst.com/sports-analytics-conference-part-2/ – (22/11/2011)

“The data revolution keeps stumbling on new truths. At Manchester City, for instance, the analysts finally persuaded the club’s then manager, Roberto Mancini, that the most dangerous corner kick is the inswinger, the ball that swings towards goal. Mancini had long argued (strictly from intuition) that outswingers were best. Eventually he capitulated and, in the 2011-2012 season, when City won the English title, they scored 15 goals from corners, the most in the Premier League. The decisive goal, Vincent Kompany’s header against Manchester United, came from an in swinging corner.”
From review of the book …
http://www.newstatesman.com/culture/2013/06/how-spreadsheet-wielding-geeks-are-taking-over-football

“Wilson recalls one particular period when Manchester City hadn’t scored from corners in over 22 games, so his team decided to analyse over 400 goals that were scored from corners. They noticed that about 75 percent resulted from so-called in-swinging corners, the type where the ball curves towards the goal. “In the next 12 games of the next season we scored nine goals from corners,” Wilson says.
http://www.wired.co.uk/magazine/archive/2014/01/features/the-winning-formula (date)

Since the first time I came across these articles, this claim struck me as poor example of the kind of useful insight that analytics can deliver, and definitely not one that should get so much attention. I also did not understand this fascination by the Man City analytics department with goals scored from corners given their low impact on the game, a fact highlighted in blogs [1]  by Chris Anderson (@soccerquant), although the year he analysed was a poor one for corners

So, when I saw this Man City’s claim recently mentioned e in Wired (see above) I could not stop myself reflecting on on the phrase, “In the next 12 games of the next season we scored nine goals from corners,” Wilson says. Nine goals from corner in the next 12 games! That can’t be right! I am aware that a few goals come from corners, so to score nine in twelve consecutive games struck me as a rather exceptional event. I decided to investigate.

Opta stats do not specify how corners are taken, so there is no way to find from their data whether this stat was true. Anyway, I only had Opta summary data for the 2011-2012 EPL season, as provided by the now defunct MCFC project. So, using Opta data was out. However, I remembered that corner type (in-swinging or out-swinging) was specified in detailed match commentaries that I had collected from the web in the past: from 2007-08 to 2011-12, when for some reason, and to my chagrin, I could no longer find them anywhere. (I am tempted to comment on the lack of free data on football beyond the few traditional match stats – but this is probably better left to another blog).

So, I extracted the relevant data from these commentaries, and produced the following charts:

MC All cornersFig. 1 – All corners
Fig. 1 shows the number of inswinging and outswinging corners taken in the years specified. It is clearly visible that more in-swinging corners are taken: 60% more on average.

MC All goalsFig. 2 – All goals from corners
Fig. 2 Show the goals scored from each type of corners. A weighted average of comers and goals, shows that on average 50% more goals are scored from in-swinging corners than out-swinging ones . I should add that I was rather puzzled by the small number of goals scored from corners in 2010-2011 (a stats which perhaps merits further investigation), but after checking and re-checking, using also published data, I had to accept that this was the case.
MC cornersFig. 3 – Man City corners
Fig. 3 shows that the mix of corner taking by Man City does not follow the general pattern shown in Fig.1 (more in-swinging than out-swinging are taken in all seasons), and instead it changes for from one season to the next. A significant change occurs in the 2011-12, when tree times more in-swinging than out-swinging corners are taken.
MC goalsFig. 4 – Man City goals from corners
Fig. 4 Man City scores very few goals from corners, with highest totals in 2009-10, and in 2011-12, when they score 9 goals, all from inswinging corners. The latter exploit is probably the most relevant statistics to keep in mind.

So, now that we have the stats, let’s look at each of the three claims reported above, in order of time.

Claim 1
The first was (supposedly) made by Gavin Fleig, MC Head of Performance Analysis at the time (Nov, 2011),. “…City had gone 22 games and not scored a goal from a corner. After this…. they scored 8 goals in 15 games“. No timeline is given for these event, but since this claim was made at a conference in November 2011, and cannot refer to a distant past, we can safely assume it fall within the range of my data. So, by looking at the charts (fig. 4), one can see that it could only have happened in the 2009-2010 season, the one preceding this claim.

During this season, according to my stats , Man City scored 7 not 8 goals (but keep in mind that I have counted only goals scored directly from corners) all season, and in the following days’ play: 4, 10, 13, 24, 32 (3). From this sequence we can see that it did not go 22 games without scoring from corners as claimed, but only 10 (14-23). Moreover, MC did not score 8 goals in the following 12 games as claimed, but only 4, of which 3 on day 32 in a 6-0 win against an already relegated Burnley, which hardly merit being included in the count.

Of course Gavin’s 22 consecutive goalless games could also include games played at the end of the previous season 2008-2009, when Man City scored only one goal from corners all season (a record?). I’ll leave readers to query that stat, but, as you‘ll remember, we are still left with the second part of the claim:” …8 goals in 15 games “, one that doesn’t tally.

Claim 2
Man City corners stats have achieved such iconic status to merit a mention in a much publicised recent book on football analytics,“The numbers game: Why everything you know about football is wrong”. I haven’t got round to read it yet, so I can only comment on what has been reported in a review, which states that in “…winning the 2011-2012 season, when City won the English title, they scored 15 goals from corners, the most in the Premier League.” Fifteen (15) goals is the official Opta figure. I only found nine (9) , and, significantly, all coming from in-swinging corners.

However, it is likely that this Opta stat includes “goals created from this particular match situation are defined here as occurring within three touches of a corner“, a rule mentioned in the already mentioned blog by Chris Anderson, one of the authors of this book, and titled “Why the Goal Value of Corners Is (Almost) Nil: Evidence from the EPL” [1]. His analysis should leave many fans wondering about the Man City analytics crowd fascination with corners.

Claim 3
Last but not least, we come to the claim, as reported in Wired, made by the top analytics man himself, Simon Wilson, Man City Strategic Performance Manager.

Wilson recalls one particular period when Manchester City hadn’t scored from corners in over 22 games, so his team decided to analyse over 400 goals that were scored from corners. They noticed that about 75 percent resulted from so-called in-swinging corners, the type where the ball curves towards the goal. “In the next 12 games of the next season we scored nine goals from corners“, writes Wired.

This article is very recent, dated 23rd January, 2014, but the claim is similar to that made by Gavin Fleig in Nov 2011 (two years earlier!). And must obviously refer to the same ‘fact’: the sequence of goalless games is the same, 22, and so is the one when goals were scored 12 (I’ll leave it to the statisticians among you to calculate the probability of this event being repeated). But the number of goals now jumps to 9, not 8 as claimed by Gavin Flieg. I have already commented on the accuracy of these stats in Claim 1, so I’ll just deal with Wilson’s other claims: that 400 goals from corners were analysed, and that 75% of these were scored from in-swinging corners.

First, though, I should point out that Wilson does not specify that the 9 goals were scored from inswinging corners, although it is clear from his premise that is what he means. But, as we have already seen, this not tally with my figures, which show that in 2009-2010 only 3 goals came from this type of corner, while 4 came from out-swinging ones – hardly a strong case for favouring in-swinging corners. But perhaps Wilson (speaking last year, I presume) is mixing the stats of 2009-10 with those of 2011-12 when 9 goals where scored – all from in-swinging corners. As to the 75% success of in-swinging claim, this does not tally with my stats that show only a 61% advantage – not an insignificant difference.

Concluding remarks
So, here you have it. My stats appear to refute Man City’s much publicised corner claim (claims?). What next?

As I started thinking of how to wrap-up my piece with some “Conclusions”, many thoughts came to mind, and began to write. But then doubt and caution won the day, and stopped.  I wondered: how could a big club like Man Cit, with claimed 11+ people working on analytics could make such an error? Surely among these there must be some with A-level stats, and knowledge of Excel (even though neither is really necessary for doing simple stats).

I hesitated, and came to the decision to leave my Conclusion to a later blog. This would give time to the varius people mentioned, as well as interested football analyst, a chance to set the record straight, and, perhaps, question my findings.

[1] http://www.soccerbythenumbers.com/2011/05/why-goal-value-of-corners-is-almost.html


Notes on data
As a professional data analyst, I am often riled by the lack of free data on many topics of wide public interest (of which soccer is perhaps the least). I believe that anyone making a public claim based on the analysis of data should be prepared to make this available to all who request it. An action that in the digital age this is rather easy to do. The capacity for other people to repeat the analysis, and verify or refute a data-bases claim is of fundamental importance. Else statistics will always inhabit that realm between science and non-science, and won’t be taken seriously.

In line with this belief, the data I used – not all, but the one specific to Man City corners – is available on request received by fellow bona fide analysts. They must keep in mind however, that this data may be subject to copyright by the original publisher, and cannot be distributed at will.

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About soccerlogic

Data analyst/miner of 23 years experience. Pretty sure I was first (1998) to apply Statistical Analysis and Machine Learning to study performance in soccer. I probably invented Soccer Analytics or, as I called it then, Football Intelligence. Haven't stop learning since, and experimenting new analysis that can help teams improve performance.
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