Friday 27 February 2015

"Penalty Kick to Chelsea!"

The use of data to inform and illuminate is gradually creeping into the mainstream media with varying degrees of success. The best examples legitimately combine excellent writing, an engaging narrative and data collection and interpretation, to produce a finished item that can be enjoyed and debated on many levels.

The Guardian's Sean Ingle currently sets the bar at a high level.

The most recent example sees Sports Writer of the Year, Martin Samuel adding a dash of data to his piece on Chelsea's penalty record in the Premier League in the Mail Online.

Mourinho has complained loudly that his team are hard done by when penalties have failed to be awarded this season and Samuel cites statistical analysis carried out by Opta to support Jose's assertions. (Although to be fair to Opta, they appear to have merely undertaken the collection part of the analysis and any interpretation of the data appears to be wholly the work of Samuel).

Samuel's approach follows a well trodden route. It uses Chelsea's rate of being awarded spot kicks in the Premier League this year (1 per 13 games in 26 matches) compared to their rate in the Champions League ( 1 per 1.8 games in 7 matches) and also against those of their English rivals from the CL and domestically.

Samuel then uses the "whopping additional 11.2 games needed to win a penalty in the Premier League" as the main "corroborative" evidence that Chelsea are getting a raw deal.

Firstly, referees appear to have particular regional traits, especially in areas such as on field discipline, where card make ups can vary considerably from one country to another. Therefore, is shouldn't be a surprise if Chelsea's CL refs, comprising Croatian, Spanish, Italian, Dutch Turkish, Norwegian and Swedish may differ in their interpretation of what constitutes a penalty kick compared to their Premier League counterparts.

Secondly, the level of competition faced by Chelsea, particularly in the CL group matches hasn't been very strong and more spot kicks tend to be conceded by teams which are vastly inferior to their opponent.

The Euro Club Index  rated the average of Chelsea's three group opponents at 2555 ranking points, the equivalent of a group made up of three teams of the average quality of a Stoke City. The easiest team from the group, NK Maribor currently rank 208th in the Index, inferior to virtually every current Premier League team.

(As an initial stage to the statistical analysis of Chelsea's seven game Champions League campaign, offered without any additional interpretation, two of Chelsea's four CL penalties came in their 6-0 defeat of Maribor).

But the main issue we should take with this mainstream article is in the use of headline penalty award rates derived from sample sizes that are unequal and limited in size.

Events do not happening in neat evenly spaced distributions. For example a fair coin which dutifully records a head followed by a tail should be treated with suspicion, rather than a confirmation that it is fulfilling an obligation to land a head or tail with equal likelihood.

Chelsea could easily have a typical Premier League likelihood of receiving penalty kicks of around one every 4.7 games and be awarded just two in 26 matches. The chances of this occurring or worse is around 7%. And the chances of any top side suffering this apparent injustice is greater still.

Relatively uncommon events will inevitably produce prolonged periods when they do not occur and others when they appear unnaturally clumped together, and this does not in itself provide evidence that a team is getting a raw deal.

You could have used a similar seven game Premier League sample to show have hard done to Manchester City were when they went into their 8th of the season match against Spurs without a penalty to their name.

90 minutes later, they had three.

Randomness is a much more likely candidate than conspiracies to explain Chelsea's record of penalty awards in 2014/15.


  1. you'd have to factor in the number of clear penalties that are not given though surely? and compare that with the number given. hard to be sure without some thorough research (this article certainly doesn't contain any_ but you have to say that two penalties given and 6+ clear penalties not given is likely to be an unusual ratio

  2. Clear penalties in the opinion of whom? Someone with a vested interest, such as Jose? I could come up with three or four "penalties" which Stoke "should" have been given this season, and a much worst performance from a particular ref, but I'd be biased.

    Samuel could have stuck with "I thought the Kightly incident was a penalty", but invoking stats that are no way "illogical" nor unusual is a poor use of the available data.