Blog Index
The journal that this archive was targeting has been deleted. Please update your configuration.
Navigation
« Problems with an Adjusted Plus Minus Metric in Football | Main | An Analysis of the Performance of Promoted Clubs »
Wednesday
Nov092011

How to Succeed in the EPL: Chances Created and Chance Conversion

A common statistic that many people have begun to value and notice a lot recently is the chances created statistic. Chances created, according to Opta's website, is defined as "assists plus Key passes" where a Key Pass is "the final pass or pass-cum-shot leading to the recipient of the ball having an attempt at goal without scoring" (Opta is a company that tracks and generates a ton of data in soccer). So basically, any pass that leads to a shot is considered a chance created.

Swansea's Mark Gower is a perfect example
of a player highlighted by the chances
created statistic.

Chances Created
The appeal of this measure is that it can value players that play on weaker teams better than assists do. For a player on a weaker team, it is harder to record assists since they are playing with teammates that are less likely to score. Chances created is a fairer statistic because it does not value the strength of your teammates as much. Overall, it can highlight creative players that are often overlooked because they are on weaker teams and do not have as many assists.

Do Chances Created Actually Matter?
With all this in mind, I was curious to find the actual worth of the chances created statistic. One way to measure this is to look at how chances created and wins are correlated. To make it a little easier, I looked at the relationship between goals scored and chances created for EPL teams. In other words, do teams that have more chances created score more? Do teams with less chances created score less? The answer, in short, is yes, they are correlated. Below is a scatterplot of the relationship. There is a clear positive relationship between chances created and goals in the EPL last season. The coefficient is statistically different than 0 (p=.000), which tells us that there is extremely strong evidence that there is a positive relationship.

Chance Conversion Percentages

This is only half the story though. Some teams get a lot of shots off, but either because they are not good at shooting or are taking shots that have a smaller chance of going in, some of these teams have a low number of goals because they have a poor conversion percentage for shots. The conversion percentage is defined as the goals divided by the total number of shots (excluding blocked shots). Below is a scatterplot similar to the one above, this time with conversion percentages on the x-axis. The conversion rates are rounded to 2 decimal places, hence the bunching. Again, this shows a positive relationship between conversion percentage and goals. Teams with higher conversion rates tend to score more and vice versa. This relationship is also statistically different from 0 (p=.002). A quick note: the product of chances created and conversion rate is very close to the number of goals a club has scored. I'm pretty sure the discrepancy comes from including blocked shots in shots attempted, but not in conversion rates.



EPL 2010-2011, Chances Created and Conversion %
With this in mind, I created a scatterplot of conversion rates and chances created for EPL teams last season. The plot shows that clubs found scoring success in different ways. The Manchester clubs did it by being efficient scorers; they had conversion percentages of .15 and .16. Chelsea and Tottenham were on the other end of the spectrum with higher chances created, but lower conversion percentages (.12 for both). The graphic also shows that West Ham did not struggle because they were not creating chances; they struggled because they had a low conversion percentage (.10). On the other hand, Birmingham struggled because they failed to create enough chances to score, despite a decent conversion percentage of .12.

EPL 2011-2012 thus far, Chances Created and Conversion %

What about this year? Below, I created the same scatterplot as above, this time for the current season. City's dominance is really highlighted. They are leading in both chances created AND conversion percentage, hence the massive number of goals this year. Again, United seems to be scoring because of their high conversion percentage. QPR and United actually have very similar number of chances created, United just finishes their chances with a much higher percentage. Liverpool sticks out because of their high number of chances created, but really low conversion percentage (.09).




Conclusion
The bottom line is that creating chances and conversion rates are the key to understanding goal scoring. A club can succeed with a high conversion rate (United) or by creating a lot of chances (Liverpool). A club can really dominate by doing both well (City). The graphic above can also suggest what kind of players each club needs. For example, Manchester United and Newcastle would benefit by picking up a creative midfielder who creates more chances, and Liverpool and QPR would benefit by picking up a more efficient scorer. The scatterplot also tells us why some clubs struggle. Wigan needs to up their conversion percentage (currently a dismal .06) and Stoke needs to create more chances. City, on the other hand, should just continue to buy all the best players.

All data comes from eplindex.com (@EPLIndex)

References (1)

References allow you to track sources for this article, as well as articles that were written in response to this article.

Reader Comments (5)

Good stuff,Ford.
Shot efficiency is a fascinating subject,with lots of competing factors.

Take Chelsea and Spurs being less efficient than the Manchester clubs last season.Last year both the London teams conceded the first goal much more often than teams of their ability would normally have done and they also trailed in matches for longer than expected.So they were attacking against teams who were possibly content to concentrate more on defence than attack.Therefore Chelsea and Spurs could have found themselves trying to convert chances under more difficult chances than usual and they could have become slightly more speculative in their attempts where they found themselves in unaccustomed losing positions.Hence they became less efficient.

The Manchester clubs were in the opposite position,they led and opened the scoring more often than even teams of their ability usually do last season.Therefore soon or later their opponents would have to commit more to attack offering United and City clearer cut chances on a more frequent than usual basis,resulting in enhanced shot efficiency.

Overall shot efficiency correlates to game time spent leading.You just have to decide which causes which :-)
I've posted about it here if you don't mind a link.
http://thepowerofgoals.blogspot.com/2011/11/shot-conversion-rates.html

nice blog
Mark

November 11, 2011 | Unregistered CommenterMark Taylor

Thanks Mark. You're definitely right, not all chances created are the same. That's probably the biggest problem with the analysis. This at least gives us a good starting point.

November 11, 2011 | Unregistered CommenterFord Bohrmann

Thanks for this article! It was great and I discussed it in my sports statistics class.

March 31, 2014 | Unregistered CommenterNate

While chance created may not be perfect and a very objective measure (there is a degree of judgment applied as to what constitutes a chance), it is still better than shots or shots on target, which are less than perfect - the quality of the shot is disregarded and chances that ended with no shot at are not taken into account. So goal chances created is definitely a useful metric. After all, we must not concentrate on and analyze what is easiest to measure but what is most meaningful and has the biggest impact. It is probably quite expensive to purchase data on goal chances from Opta.

April 16, 2014 | Unregistered CommenterNick

While chance created may not be perfect and a very objective measure (there is a degree of judgment applied as to what constitutes a chance), it is still better than shots or shots on target, which are less than perfect - the quality of the shot is disregarded and chances that ended with no shot at are not taken into account. So goal chances created is definitely a useful metric. After all, we must not concentrate on and analyze what is easiest to measure but what is most meaningful and has the biggest impact. It is probably quite expensive to purchase data on goal chances from Opta.

April 16, 2014 | Unregistered CommenterNick

PostPost a New Comment

Enter your information below to add a new comment.

My response is on my own website »
Author Email (optional):
Author URL (optional):
Post:
 
Some HTML allowed: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <code> <em> <i> <strike> <strong>