Thursday 19 January 2012

Passing Accuracy in the Premiership Corrected for Pass Length or Why Stoke are this Season's Best Passers.

Before we start to address passing percentage in the EPL,I'd like to use a short illustration from American Football. When the Houston Texans became the 32nd and most recent addition to the National Football League they earned the right to select that year's most promising prospect and they exercised that right by choosing Fresno State quarterback,David Carr.Carr spent five largely disappointing seasons at Houston before swapping the AFC Conference for the NFC where he served time as a rarely used backup most notably in New York with the Giants and in San Francisco.

Ironically it was during his final season with the Texans that Carr produced his career high numbers for passing percentage,he was successful on over 68% of his 442 attempted throws during the 2006 season.Taken in the raw,these numbers appear to be rather impressive.Acknowledged future Hall of Fame candidates such as Peyton Manning and Tom Brady,average only 64% over their most productive seasons.

 By comparison with Carr's gaudy 2006 season figures,Atlanta quarterback Michael Vick completed just 57.5% of his 2006 passes and like Carr,Vick wasn't retained by his then employers the Falcons.Vick didn't return to Atlanta in 2007 for the very good reason that he was serving jail time for aggravated animal cruelty,but despite his apparently lowly completion figures he was able to claim a starting role with the Philadelphia Eagles upon his prison release in 2009 and unlike Carr he remains a current starting player. So what made Carr's 68% completion rate unattractive to prospective employers,but Vick was able to win a mega bucks contract with his less than impressive 57.5%.Part of the answer can be found if we look at how far each player was throwing the ball through the air.In Carr's career high season he was on average throwing his passing attempts for less than 5 yards through the air,whereas Vick's passing attempts were going for over twice that distance.In short Carr was attempting relatively easy,short throws compared to Vick's longer more difficult ones.

 The issue of passing distance therefore offers some mitigation for Vick's apparently poor completion rate,but we can go much further by regressing the completion rates for all starting quarterbacks against the average distance of ground they throw the ball.We can then plug into the regression line both Vick and Carr's average throwing distances for the 2006 season to see the completion rate we would expect from an average,starting quarterback and that quickly reveals the reason for Carr's ultimate demise.A typical starting quarterback who was asked to throw the kind of short passes required of Carr in his final season at Houston would likely have a completion rate of almost 72%,so Carr's recorded 68% marks him down as a well below average completion passer.

By contrast Vick's contemporaries would have completed his depth of throws just over 55% of the time,Vick for all of his faults on and off the field was still an above average passer in 2006.

 Back to the English Premiership,where the idea of correcting passing statistics to account for length of pass hasn't yet been embraced.The longer the pass the more the opportunity for the ball to deviate from it's intended target and the greater the time available for an opponent to converge on the target or make an interception.In short,longer passes are more difficult to execute and carry an intrinsically bigger chance of failure. Team passing statistics are increasingly becoming available for the EPL,but seldom in a form that make analysis possible.However,passes are being broken down between short and long passes and that allows a way to begin to put some context on the raw numbers.(NFL passes are similarly broken down as short or deep passes and these designations can be used as a reasonable proxy for a player's actual air yards).If we use the proportion of long passes a team makes over a season as a way of classifying a team's likely average passing length and plot this against that team's passing completion percentage we immediately see that there is a very strong correlation.In general and logically the higher proportion of long passes made by an EPL team,the lower it's completion rate.

  How an EPL's Pass Completion varies with Increasing % of Long Passes.2009-12.

As with the earlier quarterback comparison,we now have a means to input a team's actual percentage of long passes into the regression line to predict the rate at which a typical EPL team would complete those passes and we can then see which teams are out performing that rate and by how much.Equally we can see teams who are under performing the league average and whose completion figures are possibly flattered by a short and safe style.I've only taken data from the last two completed and this year's part completed seasons.

Below I've listed the teams who have outperformed their predicted passing completion by most over the last three seasons.The table contains a variety of styles ranging from shorter passing sides such as Chelsea,Swansea and Man Utd to long ball merchants such as Birmingham,Norwich and inevitably Stoke.

Tottenham must claim the crown of most accomplished passing side over the period of the data.They appear three times in the top ten and are using slightly shorter passes this season,but have out performed the league average for their average pass length in all three seasons.Swansea also justify their reputation as a passing side,they don't go deep very often,but in terms of adjusted passing quality they slip in between Chelsea and Man Utd  so far this year.

Glenn Whelan prepares to launch it long.

Almost inevitably,Stoke,the bete noire of the EPL confound their critics by topping the table.Their unorthodox approach has won them few friends,but as with most things they do,they do it extremely well.They top the table for playing the largest percentage of long balls over the three seasons and that inevitably means that they will prop up the table for raw completions,but when pass length is accounted for they rise to midtable for the previous two seasons and  are the biggest over achievers this year.An average side would complete just under 65% of Stoke like passes,Stoke manage almost 70%.Visually I feel much of their improvement this year is down to the addition of Crouch.His ability to hold onto forward passes,whilst holding off defenders is most impressive and highlights another flaw in raw passing statistics.Namely you have to pass to someone and the better the quality of the recipient,the easier it is for a passer to accumulate good raw passing stats.

Largest Over Achieving Passing Teams when Correcting for Pass Length. 

Team. Year % of  Long Passes Pass Success % Predicted Pass Success% % Difference
Stoke. 2011/12 23.5 69.8 64.7 +7.9
Tottenham. 2011/12 14.2 84.8 80.2 +6.0
Blackpool. 2010/11 19.3 76.4 72.1 +5.9
WBA. 2010/11 18.0 77.9 74.3 +4.8
Birmingham. 2010/11 21.4 71.6 68.5 +4.6
Tottenham. 2009/10 18.6 76.6 73.2 +4.6
Norwich. 2011/12 19.7 74.4 71.3 +4.3
Tottenham. 2010/11 16.4 80.1 77.0 +4.1
Wigan. 2009/10 19.3 74.8 72.1 +3.8
Wigan. 2011/12 15.5 81.1 78.3 +3.6
Chelsea. 2011/12 11.8 85.6 82.8 +3.4
Swansea. 2011/12 12.0 85.2 82.6 +3.1
West Ham. 2010/11 18.5 75.7 73.4 +3.1
Man Utd. 2011/12 12.9 84.1 81.6 +3.0

For every winner there's losers and a similar process can identify which teams should be completing a higher percentage of their passes given their chose of pass length.Fulham and Blackburn clog up the top five,although as a slight encouragement they don't appear in this season's guise.Perhaps it was a problem that was identified and addressed in the close season,both teams are hitting slightly less long balls this year,so maybe they were particularly bad when going deep.Sunderland,Wolves and Bolton also feature prominently,but the presence of Arsenal's 2009/10 side and Liverpool's 2010/11 team will perhaps surprise those who judge solely by reputation.

Largest Under Achieving Passing Teams when Correcting for Pass Length. 

Team. Year % of  Long Passes Pass Success % Predicted Pass Success% % Difference
Fulham. 2009/10 14.7 74.3 79.4 -6.4
Blackburn. 2009/10 21.1 64.5 68.9 -6.4
Blackburn. 2010/11 21.2 65.2 68.8 -5.2
Aston Villa. 2010/11 15.6 74.1 78.1 -5.2
Fulham. 2010/11 14.4 76.2 79.8 -4.5
Liverpool. 2010/11 14.1 77.1 80.2 -3.9
Sunderland. 2010/11 18.0 71.8 74.4 -3.5
Sunderland. 2009/10 21.1 66.7 68.9 -3.2
Wolves. 2010/11 17.8 72.3 74.6 -3.1
Wolves. 2009/10 20.3 68.2 70.4 -3.1
Bolton. 2010/11 21.0 67.0 69.0 -2.9
WBA. 2011/12 14.5 77.3 79.6 -2.9
Arsenal. 2009/10 10.9 81.4 83.6 -2.6
Bolton. 2009/10 22.8 64.2 65.8 -2.5

These figures shouldn't be used to endorse long passing over short passing or vice versa,unlike the NFL where passing is pre eminent passing is but one component of football (or soccer to avoid confusion).The numbers merely show that team's can and do chose to mix up their proportion of long and short passes and it's very likely that some teams are better at completing different lengths of passes than other teams.

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