Evaluating the abilities of a football team can take many forms. From a purely numerical analysis, where their strengths and weaknesses are expressed as lines on a spreadsheet and future performance is cited in probability, to gut instinct formed through actually watching a side interact and play the sport in the flesh, complemented with a knowledge of their recent player acquisitions and departures.
Neither preference is guaranteed to capture the complete nuance of each side's true worth. So can an integrated approach that borrows from both sides of the divide provide a worthwhile collaboration?
In this guest post I try to combine the strengths of both approaches.
Sunday, 17 November 2013
Friday, 8 November 2013
Conversion Percentage Inside the Box. Mind The Gap.
So I used 2011/12 seasonal shot data to look at all goal attempts to see if the rate of scoring was merely the result of random variation around a common mean or if their was likely to be a genuine difference between the best and the worst sides in terms of conversion efficiency.
I split the sample between shots inside the box and shots from everywhere else (including shots and goals scored from you own box.....Tim Howard, take a bow), in an attempt to maintain a decent sample size, but to also smooth out any positional shooting preferences among the teams.
The method sees if the distribution of goals scored by each side, given their relative shot totals, is substantially different from the range you may expect if every side is equally talented at converting similar types of chances.
I deliberately left in penalty kicks for shots from inside the box, because I wanted to see how the conclusions changed as certain types of readily identifiable and unevenly distributed shots were removed from the sample.
So the first run included every goal attempt from inside the box. The distribution of goals scored by the twenty sides did appear to differ markedly from the spread you might expect to see if Manchester City had had 450+ attempts and Stoke had 230+ with every other team contained somewhere between those shooting extremes, but all sides had striking talent that was equally adept at converting the chances that fell to them.
Next, I took out penalties, which tend over time to be given to those that do the most attacking and present a significantly higher chance of scoring that other, open play opportunities from inside the box.
Virtually the same result.
Compared to the sample with penalties, we do edge very slightly closer to a distribution of actual goals in 2011/12 that better resembles a random draw from an equally talented 20 team strike force being presented with varying numbers of opportunities. But we still can very safely say that our actual spread of goals from 2011/12 doesn't resemble a lucky dip with a universal strike rate. About 2% of teams manged at least 60 goals from the distribution of shots actually attempted by teams during 2011/12 in simulations using a universal, average conversion rate. In reality during 2011/12, three teams out of 20 managed to surpass this target.
So I then took out headers.
Overall, headers present a poorer likelihood of success compared to shots and in 2011/12 headers comprised a heft chunk of the total goalmouth attempts for some teams, (no prizes for guessing Stoke).
With headers culled from the data, the difference between the actual distribution and the range you might expect from one drawn from a group of equally lethal strikes, plummeted to within touching distance of each other.
It is just one season, but once you take out penalties and headers, then the number of goals scored by all other means inside the box, still differs from what might expect to occur by random chance where there is no difference in the finishing talents of each forward line, but the gap is small....Very small.
Here's the regressed conversion rates for shots (with the feet) inside the box for sides from 2011/12 suggested by the above analysis.
To put these figures into perspective, the difference in conversion rates between top and bottom, given an average number of shots from inside the box (240) accounts for 4 extra goals and that represents about three league points.
If we ignore Newcastle at the top and Liverpool at the bottom, both of whom broke most statistical models during 2011/12, the actual top five from 2011/12 are to be found in the top seven for converting shots inside the box. And relegated Bolton and Blackburn are at least in the bottom half. So the ranking is fairly consistent with league position in May.
By attempting to produce a reasonably sized, homogeneous sample size, the gap between the degree by which real life conversion rates fall, at first slightly and then precipitously towards a random draw is seen. There's still evidence for a talent divide at the very top, but it is narrowing, throwing the importance of shot volume into the spotlight.
Ten years worth of shot data would be nice to see which side of the line shot conversion rates finally settle on!
I split the sample between shots inside the box and shots from everywhere else (including shots and goals scored from you own box.....Tim Howard, take a bow), in an attempt to maintain a decent sample size, but to also smooth out any positional shooting preferences among the teams.
The method sees if the distribution of goals scored by each side, given their relative shot totals, is substantially different from the range you may expect if every side is equally talented at converting similar types of chances.
I deliberately left in penalty kicks for shots from inside the box, because I wanted to see how the conclusions changed as certain types of readily identifiable and unevenly distributed shots were removed from the sample.
So the first run included every goal attempt from inside the box. The distribution of goals scored by the twenty sides did appear to differ markedly from the spread you might expect to see if Manchester City had had 450+ attempts and Stoke had 230+ with every other team contained somewhere between those shooting extremes, but all sides had striking talent that was equally adept at converting the chances that fell to them.
Next, I took out penalties, which tend over time to be given to those that do the most attacking and present a significantly higher chance of scoring that other, open play opportunities from inside the box.
Virtually the same result.
Compared to the sample with penalties, we do edge very slightly closer to a distribution of actual goals in 2011/12 that better resembles a random draw from an equally talented 20 team strike force being presented with varying numbers of opportunities. But we still can very safely say that our actual spread of goals from 2011/12 doesn't resemble a lucky dip with a universal strike rate. About 2% of teams manged at least 60 goals from the distribution of shots actually attempted by teams during 2011/12 in simulations using a universal, average conversion rate. In reality during 2011/12, three teams out of 20 managed to surpass this target.
So I then took out headers.
Overall, headers present a poorer likelihood of success compared to shots and in 2011/12 headers comprised a heft chunk of the total goalmouth attempts for some teams, (no prizes for guessing Stoke).
With headers culled from the data, the difference between the actual distribution and the range you might expect from one drawn from a group of equally lethal strikes, plummeted to within touching distance of each other.
It is just one season, but once you take out penalties and headers, then the number of goals scored by all other means inside the box, still differs from what might expect to occur by random chance where there is no difference in the finishing talents of each forward line, but the gap is small....Very small.
Here's the regressed conversion rates for shots (with the feet) inside the box for sides from 2011/12 suggested by the above analysis.
| EPL Side from 2011/12. | Regressed Conversion Rate for Foot Shots Inside the Box %. |
| Newcastle. | 14.9 |
| Arsenal. | 14.8 |
| Chelsea. | 14.8 |
| Manchester United. | 14.7 |
| Norwich. | 14.6 |
| Tottenham. | 14.4 |
| Wolves. | 14.3 |
| Manchester City. | 14.2 |
| Aston Villa. | 14.1 |
| QPR. | 14.1 |
| Stoke. | 14.0 |
| Sunderland. | 14.0 |
| Bolton. | 14.0 |
| Everton. | 13.9 |
| Blackburn. | 13.9 |
| Swansea. | 13.9 |
| Fulham. | 13.8 |
| WBA. | 13.8 |
| Wigan. | 13.2 |
| Liverpool. | 13.1 |
To put these figures into perspective, the difference in conversion rates between top and bottom, given an average number of shots from inside the box (240) accounts for 4 extra goals and that represents about three league points.
If we ignore Newcastle at the top and Liverpool at the bottom, both of whom broke most statistical models during 2011/12, the actual top five from 2011/12 are to be found in the top seven for converting shots inside the box. And relegated Bolton and Blackburn are at least in the bottom half. So the ranking is fairly consistent with league position in May.
By attempting to produce a reasonably sized, homogeneous sample size, the gap between the degree by which real life conversion rates fall, at first slightly and then precipitously towards a random draw is seen. There's still evidence for a talent divide at the very top, but it is narrowing, throwing the importance of shot volume into the spotlight.
Ten years worth of shot data would be nice to see which side of the line shot conversion rates finally settle on!
Thursday, 7 November 2013
Begovic Scores! We Draw!
All of the best photographers appear to have a sixth sense about when a worthwhile picture opportunity is about to arise. It was therefore no surprise that my camera had just been returned to my rucksack when Asmir Begovic (a goalkeeper) scored for Stoke (after 13 seconds) against Southampton at the Britannia Stadium on Saturday.
There were clues available that would have indicated that Mark Hughes had a plan. All teams have a preferred end to attack in the second half and there appears to be a tacit agreement between captains that if the visitor wins the coin toss, he will take the kick off, rather than earn the early game wrath of the home supporters by turning the teams around. Therefore, the undercurrent of discontent that accompanied the sides swapping ends after the coin toss on Saturday, turned to slight bemusement as Southampton lined up to take the kickoff.
Stoke had turned themselves around.
The geography of the Britannia Stadium makes it an ideal site for endurance training. The prevailing wind regularly blows in from Trentham, funnels itself through the two open corners at the south end of the ground and then struggles to exit at the single open corner to the left of the Boothen End, tipping a hat to the statue depicting the three ages of Sir Stan as it carries on towards the city.
Stoke now have a chequered history with near gale force winds. In the distant past it has removed the roof of the Butler Street Stand (along with our best striker to pay for the uninsured infrastructure), but the worst it has managed at the Britannia was the late postponement of a game against WBA. On Saturday it provided Stoke with the opening goal by way of partial payback.
In the subsequent press conference, Mark Hughes acknowledged the deliberate decision to play with the wind during the first half, citing the importance of scoring first, which is encouraging. However, it is (hopefully) unlikely that his keeper was considered the most likely scorer. Hughes' apparent encouragement for his sides to shoot from distance, so vividly demonstrated at QPR, must have some bounds.
Following the kick off, Southampton chose to attack Stoke with a series of intricate passes. This quickly broke down, the ball was rolled back to Begovic, who launched a wind assisted punt goalwards. Both Southampton defenders chose to ignore the golden rule of defending, namely "never, ever let the ball bounce" and Boruc was left embarrassed by a slick bounce on the wet surface.
Begovic is the fifth keeper to score in the Premiership and invariably such goals require additional help or unusual circumstances. Tim Howard's effort against Bolton was a replica of Begovic's goal, but Peter Schmeichel, when playing for Villa opened the goalkeeping Premeiership goal tally from the more advanced position of the opposing penalty area. So modelling the likelihood of a keeper netting is going to be hugely situational.
We may have more luck trying to quantify the quickfire timing of the goal.
The chance of a goal being scored increases slowly, but inexorably as time elapses as caution and fitness, gives way to adventure and fatigue. Your chances of seeing a goal during the sixty seconds that comprise the 8th minute is only 80% of your chances of seeing one in the 80th.
However, three "sixty second" intervals are completely atypical compared to this gradual cranking up of goal expectation. The 45th is the second most goal laden "minute" followed by the 90th and the reasons are clear. Injury time extends both minutes well beyond sixty seconds leading to two big spikes. Therefore these increased rates are purely artificially created by traditional timing considerations. The second half starts with the first second of the 46th minute, even if the first half stretched well beyond 45 minutes of actual time.
The barren blip that comes with the first minute, by contrast is entirely real. If you chose any sixty second period in the first ten minutes, you are likely to see around 0.7 to 0.8 percent of the total goals scored during the match, on average. But if you plump for the first 60 seconds of a match, you will be lucky to see much more than half of the typical early minutes goal percentage.
Again, the reasons are fairly plain to see. Every game starts with a kickoff. The ball is about as far from either goal as it can possibly be and all eleven players are positioned between the ball and their goal and this formation of maximum protection for each goalmouth is guaranteed to occur during the first minute in every match. Hence the scoring is not only at its lowest because of the usual ebb of intent and desire to score, it is atypically lower because of the requirement to start the game with a kickoff.
So, pulling all the information together, around 0.5% of goals come in the first minute, the first 13 seconds are likely to see less than a pro rata division of this goal expectation because of the guaranteed safe starting position for the ball. A back of the envelop calculation using these figures and the average scoring expectation over the history of the Premiership gives an average goal expectation for the first 13 seconds of a Premiership match of around 0.0015 of a goal. The chances of scoring twice or more in 13 seconds is impossible, therefore a goal in the opening 13 seconds should, under these informed gu-estimations happen around once every 666 matches.
So, unlikely as Begovic's goal was purely from a timing perspective on that particular Saturday afternoon, we should expect to have seen around a dozen such goals scored before the 14th second has elapsed over the 8,326 game history of the Premiership.
Begovic's strike was the sixth such effort, so maybe the primeval order at kickoff takes slightly longer to descend into chaotic normality than I accounted for or Premiership audiences have just been slightly unlucky.
If you have to miss a minute of a match and you want to reduce your chances of missing a goal, chose the first minute, (although you may be really unlucky a miss club history in the making), but at least on Saturday any tardy spectator got to see a match featuring two keepers whom had both scored a career goal, (although Boruc's strike came from the altogether more likely source of the penalty spot).
There were clues available that would have indicated that Mark Hughes had a plan. All teams have a preferred end to attack in the second half and there appears to be a tacit agreement between captains that if the visitor wins the coin toss, he will take the kick off, rather than earn the early game wrath of the home supporters by turning the teams around. Therefore, the undercurrent of discontent that accompanied the sides swapping ends after the coin toss on Saturday, turned to slight bemusement as Southampton lined up to take the kickoff.
Stoke had turned themselves around.
The geography of the Britannia Stadium makes it an ideal site for endurance training. The prevailing wind regularly blows in from Trentham, funnels itself through the two open corners at the south end of the ground and then struggles to exit at the single open corner to the left of the Boothen End, tipping a hat to the statue depicting the three ages of Sir Stan as it carries on towards the city.
Stoke now have a chequered history with near gale force winds. In the distant past it has removed the roof of the Butler Street Stand (along with our best striker to pay for the uninsured infrastructure), but the worst it has managed at the Britannia was the late postponement of a game against WBA. On Saturday it provided Stoke with the opening goal by way of partial payback.
In the subsequent press conference, Mark Hughes acknowledged the deliberate decision to play with the wind during the first half, citing the importance of scoring first, which is encouraging. However, it is (hopefully) unlikely that his keeper was considered the most likely scorer. Hughes' apparent encouragement for his sides to shoot from distance, so vividly demonstrated at QPR, must have some bounds.
Following the kick off, Southampton chose to attack Stoke with a series of intricate passes. This quickly broke down, the ball was rolled back to Begovic, who launched a wind assisted punt goalwards. Both Southampton defenders chose to ignore the golden rule of defending, namely "never, ever let the ball bounce" and Boruc was left embarrassed by a slick bounce on the wet surface.
Begovic is the fifth keeper to score in the Premiership and invariably such goals require additional help or unusual circumstances. Tim Howard's effort against Bolton was a replica of Begovic's goal, but Peter Schmeichel, when playing for Villa opened the goalkeeping Premeiership goal tally from the more advanced position of the opposing penalty area. So modelling the likelihood of a keeper netting is going to be hugely situational.
We may have more luck trying to quantify the quickfire timing of the goal.
 |
| Stoke v Southampton, two minutes 13 secs from history being made (not shown). |
However, three "sixty second" intervals are completely atypical compared to this gradual cranking up of goal expectation. The 45th is the second most goal laden "minute" followed by the 90th and the reasons are clear. Injury time extends both minutes well beyond sixty seconds leading to two big spikes. Therefore these increased rates are purely artificially created by traditional timing considerations. The second half starts with the first second of the 46th minute, even if the first half stretched well beyond 45 minutes of actual time.
The barren blip that comes with the first minute, by contrast is entirely real. If you chose any sixty second period in the first ten minutes, you are likely to see around 0.7 to 0.8 percent of the total goals scored during the match, on average. But if you plump for the first 60 seconds of a match, you will be lucky to see much more than half of the typical early minutes goal percentage.
Again, the reasons are fairly plain to see. Every game starts with a kickoff. The ball is about as far from either goal as it can possibly be and all eleven players are positioned between the ball and their goal and this formation of maximum protection for each goalmouth is guaranteed to occur during the first minute in every match. Hence the scoring is not only at its lowest because of the usual ebb of intent and desire to score, it is atypically lower because of the requirement to start the game with a kickoff.
So, pulling all the information together, around 0.5% of goals come in the first minute, the first 13 seconds are likely to see less than a pro rata division of this goal expectation because of the guaranteed safe starting position for the ball. A back of the envelop calculation using these figures and the average scoring expectation over the history of the Premiership gives an average goal expectation for the first 13 seconds of a Premiership match of around 0.0015 of a goal. The chances of scoring twice or more in 13 seconds is impossible, therefore a goal in the opening 13 seconds should, under these informed gu-estimations happen around once every 666 matches.
So, unlikely as Begovic's goal was purely from a timing perspective on that particular Saturday afternoon, we should expect to have seen around a dozen such goals scored before the 14th second has elapsed over the 8,326 game history of the Premiership.
Begovic's strike was the sixth such effort, so maybe the primeval order at kickoff takes slightly longer to descend into chaotic normality than I accounted for or Premiership audiences have just been slightly unlucky.
If you have to miss a minute of a match and you want to reduce your chances of missing a goal, chose the first minute, (although you may be really unlucky a miss club history in the making), but at least on Saturday any tardy spectator got to see a match featuring two keepers whom had both scored a career goal, (although Boruc's strike came from the altogether more likely source of the penalty spot).
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