Sunday, 19 February 2012

Avoiding Costly Errors in the EPL ?

Juande Ramos' Tottenham started their 2008/09 campaign hoping to build on their mid table finish from the previous year and they showed their intent early by completing the signing of Luka Modric .However, the ongoing saga of whether or not Berbatov would even consent to pull on a Spurs shirt would drag on almost to the end of the summer transfer window and the Bulgarian eventually followed Robbie Keane out of White Hart Lane.As £50 million of attacking talent departed it was left to Darren Bent who had been less than prolific during the previous year to carry the forward line and his first half injury time,point claiming strike at Stamford Bridge was the highlight of Tottenham's early season.

The low points,though were many and varied.Defeats to North East giants Sunderland and Middlesbrough preceded the point at Chelsea.A solitary point from the visits of Villa and Wigan merely heralded further defeats at the hands of Portsmouth,Hull and yet another in disgrace at Stoke where Spurs left the field with just nine men.Inevitably,Ramos was gone,the Potteries chants of "sacked in the morning" proving premature by only five days and the Redknapp era began with a 2-0 win at home to Bolton.

By May Spurs were placed comfortably in the top half of the table with a tally of 51 points,Bent had forced his way into the England squad and Harry's revolution that would take Spurs to the Champions League was gaining momentum.They finished the season with a goal difference of precisely zero and their actual points haul was within a point of the average expectation for a team who had scored as many goals as they had conceded.Of more interest was Spurs' unwanted record of conceding 14 goals directly as a result of individual errors.This compared to the league average of  just six goals allowed in such a way and in an environment where small margins matter it was an obvious source for improvement.If Spurs had been merely average they would have saved themselves around 8 goals,improved their goal difference and likely picked up an extra seven or eight league points.Enough to see them rise from the basement in September to Europe come May.

The 2009/10 season was Redknapp's first full season in charge and with his squad and hierarchy now in place,the England manager in waiting broke the Champions League monopoly by qualifying for the playoff stages of the tournament with a top four finish.Scoring goals played a big part in Spurs' achievement,but the previous year's habit of handing the opposition cheap goals through costly mistakes didn't go unnoticed.14 such strikes in 2008/09 would have cost Spurs their lucrative route into Europe had their carelessness been repeated,but it wasn't and instead they cantered home with an above average tally of only three such error strewn concessions.

A great story of attention to statistical detail,corrective action and a positive outcome.........except the plausibly seductive narrative is almost certainly a gross distortion of what actually occurred.

Errors that lead directly to goals are relatively rare events,from 2008/09 to 2010/11 the average number conceded by the Premiership sides was a shade over six.Tottenham's 14 from 2008/09 was the highest seasonal total recorded,although Arsenal could be on course to top that total this year.Teams such as Wigan and Hull also recorded similar numbers as their higher ranked compatriots,but the likes of Wigan again and West Ham also impressed with very low totals over the last four years.So these are a mixed bunch of results,with very little obvious pattern and viewed as a group the figures tend to resemble the seasonal distribution of draws.We saw here that draws are predominately random events where team quality has little bearing on the amount of draws a team gains over the year and season to season correlation is extremely weak.

To see if this was the case with errors leading to the concession of goals I plotted the totals in one season for all EPL teams and paired them will their records from the next season.

The scatter plot is very random,a high number of such goals in one season does not automatically lead to a similarly high number being given up during the next one.The near horizontal line of best fit that runs close to the seasonal average indicates that no matter how high or low the goals tally was in a previous season,the best guess for the next would be to regress towards the mean.We can further confirm this by taking a basket of teams who were particularly good and particularly bad at avoiding the concession of goals from errors in one season and averaging their combined totals in the next.In both cases the outliers from the previous season are dragged much closer to the average for the league in the subsequent season.

How the Best and Worst Error Strewn Teams Perform in Subsequent Years.

Average Goals Allowed from Errors for Best Performers in Season N. Average Goals Allowed from Errors for Same Teams in Season N+1.
2.4 6.4

Average Goals Allowed from Errors for Worst Performers in Season N. Average Goals Allowed from Errors for Worst Performers in Season N+1.
10.7 5.6

Given these initial results,the avoidance of goal yielding errors would not appear to be a largely repeatable skill.Teams who have the "skill" one year often lose it the next and become merely average.Also more talented teams appear to be no better at  displaying the trait than do poorer ones because if you plot the correlation between seasonal success rate and goals allowed through errors is also virtually non existent.

One of the most dangerous assumptions to make in analysis of football is that what happens on the field is almost entirely down to the talent of the players on the pitch.Many outcomes during the game are predominately talent based,but all are determined in varying degrees by a randomness that is beyond the control of the on field participants and goals scored from errors would seem to be a prime candidate for a statistic that is primarily driven by chance.These goals affect and partly explain a team's final league standing,but are poor indicators of how teams will subsequently perform in this aspect of the game in the future.

In short,teams cannot escape the random nature of mistakes,nor can they control the outcomes of those mistakes.One season almost every slip up might lead to a goal,while other seasons may simply see most of these chances being ballooned over the bar by the opposition.All that EPL teams can expect,judged on the distribution of the data is that over time they will each concede an average of around six goals in such circumstances.

The manipulation of the new raft of data is fraught with difficulty and just as players find it difficult to accept that their skill levels aren't the final arbiters of the ultimate game outcome,a new breed of number crunchers may have difficulty in accepting that a future event cannot be predicted with any degree of accuracy.Sometimes,as would appear the case here,the best guess is merely an average from all teams over previous seasons.

To return to Tottenham's horrendous "luck" where errors led to goals in 2008/09.Every player bar one whose mistakes contributed to those 14 goals returned as Spurs players in 2009/10 when just three mistakes led to goals.It's easy to envisage extensive pre season sessions aimed at "cutting out the errors" and when that was precisely what occurred it's easy to think that the therapy worked and that players can be taught to be less careless.However,the most likely explanation was that Spurs would have seen less mistakes leading to goals through natural regression towards the mean anyway in 2009/10 and the talent that serves players well in making them professional footballers equally serves them in avoiding costly errors....whatever the short term numbers say.

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