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Saturday, 25 October 2014

Subbing Your Striker.

Substitutes provide a fascinating look into the changing dynamics of a football game.

Scoring accelerates as the game progresses and in this post from 2012 I looked at the tag of "super sub" that had become attached to Edin Dzeko and how it owed much to the higher goal scoring environment in which he commonly played.

Individual players may tend to produce elevated scoring rates as a substitute not only because of the higher goal environment, but also because they will usually be playing against a minimum of seven tired opponents. Playing time for substitutes also tend to result in smaller sample sizes, leading to extremes of good or bad scoring rates.

Small sample sizes will inevitably throw up prodigious scoring rates for individual players, whereas those which inevitably fall well below normal scoring rates will tend to be neglected. The former, high scoring group of players will therefore often be used to represent the scoring feats of substitutes as a whole.

To attempt to remedy this, it seems sensible to firstly compare the records of all starting strikers who play for the entire game, all those who are subbed out and all those who take their place, to see if there is the expected benefit from playing exclusively during the later minutes of a match.

Peter Odemwingie looks forward to a period of elevated match scoring.
These results are taken from the 2011/12 EPL season and are restricted to players who were designated as out and out strikers. I looked first at the scoring rate per 90 minutes for the three different groups of strikers, as well as their conversion rates.

Striker. Goals Per 90+ Time Allowed. Goals Per Attempt.
Plays Entire Game. 0.356 0.126
Subbed Out. 0.346 0.139
Subbed In. 0.387 0.113

The average amount of playing time for the subs in this sample was 18 minutes, added time probably stretches this to 21 minutes. And as suspected, as a group they score at rates that are above either those of the strikers who were substituted and those whom played the entire game.

However, there are a multitude of factors that may alter the scoring rate of this minority group of strikers. They may be thrown onto the pitch in place of a midfielder to help chase an game, which may lead to them scoring at a high rate than usual, even allowing for the late stage of the game. But additional goals may be ceded at the other end.

Similarly, strikers subbed out of a match may have been replaced by a defensive midfielder, to help close out a game where more goals and the possibility of an increased scoring rate were there for the taking.

Allowing for these micro details of game state and managerial intention would require painstaking analysis of each individual match, but we can perhaps design a proxy to reduce the effect of goal environment and these other variables.

There does appear to be some method behind substitutions in the EPL. In this post I suggested that younger, less experienced players are proportionally substituted out of a game more frequently than more experienced players, even with such likely factors as a fitness advantage.

So there may be perhaps a systematic overall approach from EPL managers to substitutions.

Therefore, I looked just at substitutions where a striker replaced another striker, matched the pairings together and treated the combined statistics of the departing player and the newly introduced substitute as those of a single 90  minute + injury time playing event. Albeit made up of two different individuals.

This partly eliminated occasions where a side was aggressively attacking their opponents. If we compare this composite "player" combined from a subbed out and subbed in striker to a striker who plays for the entire game, we might see if the manager is getting optimum return from his ability to make substitutions by looking at like for like changes.

There were just over 300 occasions where a striker replaced a striker in 2011/12. We could speculate that more often a less talented striker was replaced by another less well thought of attacker, but with fresher legs, while the side's premier striker remained for the entire 90 minutes.

The "subbed out/subbed in" group of matched attacking players scored 115 goals in 309 matches of 94 minutes allowing for injury time. A rate of 0.356 goals per 90 minutes.

Strikers who played the entire 94 minutes, scored 277 goals in 746 games. Also a rate of 0.356 goals per 90 minutes.

Either through accident, design, a quirk of this data set or a combination of all three, in 2011/12 EPL managers were able to get goal scoring returns from a substitute striker and the striker he replaced that were identical to those returns from a striker who was considered worthy of the full 90 minutes, under broadly similar match conditions.

A case of expertise and experience eking the optimum return from a side's strike force?

Friday, 17 October 2014

Your Biggest Opponent Might Be Your Teammate.

In attempting to answer the question “how good is a striker?”, the focus unsurprisingly falls on their goal scoring abilities. Raw goal scoring totals have been replaced by such statistics as “goals per 90 minutes” to reflect playing opportunity, often with corrections to remove penalty kicks and to allow for the different scoring environments that are present later in a game.

Latterly, actual goals have been replaced by expected goals, based on the likelihood that an average player would score with a shot or header from a particular location on the pitch. 

The ability to get into good scoring positions may be as important as the successful execution of a chance.

Random variation may in the short term produce actual goal tallies that flatter or under value a player and so his “virtual” goal per minute numbers may be a better indicator of his likely long term scoring potential.

The difference in talent levels at the very top level of finishing is likely to be very small and the ability to outwit the defence and fashion a chance may be more important than short term goal scoring achievements.

The opposing defence is quite naturally considered the natural opponent of a striker trying to amass expected goal value, but often there is also another factor that might cause his numbers to rise or fall.

Liverpool was fortunate to be able to call on two exceptional strikers, last season. Both Suarez and Sturridge reached impressive, respective goal totals, both real and expected. In addition, both missed playing time for a variety of reasons. But Liverpool was able to call on at least one of the pair for every Premiership game.

Suarez’s suspension left the striking stage clear for Sturridge at the start of the season and then the roles were then reversed when the latter missed a run of matches from early November to mid-January.

While in tandem, the two strikers could possibly be competing for the best chances created by their teammates, while in the other’s absence, each may have laid claim to the majority of the prime opportunities.

Therefore, I looked at each goal attempt made by Suarez and Sturridge over the 2013/14 Premiership season as defined by shot location and type. And, reflecting the wider picture, these primary parameters were significant indicators of whether a goal was likely or not to be scored.

I then added a variable to differentiate between when both Suarez and Sturridge were on the pitch and when just one of the strikers was present. This new variable was also a significant indicator of likely success, reducing the likelihood of a goal being scored when the duo were playing together.

In the case of Sturridge, his expected goal per game number falls from 0.9 when he flies solo to 0.67 when he partnered Suarez. A fall of nearly 30%.

The same is seen for Suarez, 1.1 expected goals per game alone and 0.75 when paired with Sturridge. Once again, this is a fall of around 30%.

We are looking at nearly 300 total attempts, but the results may just be a quirk of this dataset. However, on a shot by shot basis, it does appear that the players are taking a bigger proportion of low expectation attempts and fighting over the prime cuts when both are playing.

Suarez’s average goal expectation per attempt fell from 0.23 to 0.14 when Sturridge joined him on the field. Sturridge’s comparative figures went from 0.27 to 0.18 per attempt.

This is a single case involving two extremely high class finishers and there are at least eight other outfielders to consider, but it perhaps seems that the pair as a duo may have been shooting sometimes, when the position was too heavily defended.

This may have had implications for Liverpool’s strategy when both played, although it is now a moot point.

Perhaps more pertinently, each player appears to depress the expected goal record of the other by their considerable presence on the field. And this may have implications for player assessment as well as projection at future clubs, where the most difficult opponent to overcome in posting impressive scoring statistics may well be your teammate. 

Luis Suarez debuts for Barcelona in the near future. 

Friday, 10 October 2014

Chelsea Impressive Start to the Season.

Chelsea have been impossible to ignore this season. They have either been bullying teams in their own six yard box or jostling for space in their own technical area.

Either way they have been posting extremely impressive raw shooting data and even when their relatively benign strength of schedule is accounted for, they are the Premiership's best performing side to date.

And when you additionally incorporate the distribution of big chances they are creating they pull even further ahead of their challengers.

The strength of schedule is one of the issues looked at in this post http://www.pinnaclesports.com/en/betting-articles/soccer/epl-shot-analysis-week-7?ito=twitter  

And their slew of big chance are put under the spotlight here

http://www.bettingexpert.com/blog/chelsea-quality-chances