#19: How to quantify the prevention of a potential goal scoring opportunity

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Chances Created and Chances Missed

Chances Created is a metric which tries to quantify the number of goal scoring opportunities that a player is directly involved in.
Opta definition is Assists + Key Passes, where Assists are passes (final touch) which result in a goal from the subsequent play and KP are passes which result in a shot that doesn’t become a goal.
So chances created can be reduced to the final passes (touches) before another player has an attempt at goal (and scores or doesn’t).
As I’ve discussed on podcast The Monthly Football Podcast, Assists alone are pretty random since you rely on the shot actually going in the goal, so chances created is a bit less noisy and should more reliably predict future assists than assists actually do due to the sheer volume of chances created and opportunities for goals to be scored rather than relying on goals actually being scored which is hard.

Chances created relies on a player to actually have a shot at the end, otherwise there is no record of the opportunity. Opta also have ‘Chance Missed’, which is defined as a big chance opportunity where the player doesn’t get a shot away. Chance missed will be attributed to the player who has the big chance and decides not to shoot, which doesn’t help the creator who provided the chance. If we assume that the miss is largely due to the player not executing an attempt, then mapping these chances missed back to the creator in addition to chances created would give credit to creating the opportunity and not punishing them for something out of their control, such as the forward deciding to delay a shot and missing the chance to.

Chances Denied Metric

As usual, chances created and most quantified statistics deal with the offensive side of the game since it’s more tangible. Shots are there and they happen, counting them is pretty straightforward. A bit less straightforward is to count the passes prior to shots, with chances created. Both of these can be tracked over many events and quantify expected outcomes based off similar situations in the past, this results in expected goals and expected assists. What is not straightforward is how to quantify the benefit of defensive actions.

We can count tackles, blocks, interceptions and recoveries, however, much like steals, blocks and rebounds in the NBA, they don’t quite tell the whole story about how a defence works. Weaker teams are asked to defend more since they have less possession, this means they have more chances to rack up interceptions, tackles and recoveries. Possession adjusting these measures helps somewhat to normalise these differences, which means that we can compare the frequency of each action assuming they all have equal chances to do so. However it’s still hard to differentiate the quality of the actions, or how important they were to each team.

Chances denied are an attempt to quantify how much of an opportunity was denied by an interception or ball recovery. In a purely defensive, denying your opponent a goal scoring opportunity, sense, recovering the ball in the middle of the pitch is not as important as recovering the ball on the edge of your own box. Expected threat, created by Karun Singh (@karun1710), is a metric which quantifies how likely a team is to score from each location on the pitch within the next 5 actions. If we assign the xT to a recovery or interception or tackle considering the location on the pitch it occurs then we may get a proxy for how important each action was. Since defensive teams will get more opportunities, it may be worth possession adjusting this also to compare like for like.

The general concept trying to be captured here is to quantify the quality of chance or potential quality of chance that is denied due to the action taken by the defender. This quantity can be given solely to the defender making the action or collectively assigned to the players involved to appreciate the team aspect of defensive play. There is a question whether to include tactical fouls in here as well as legal ball recoveries, but will save that for another time.

@TLMAnalytics

https://www.optasports.com/news/opta-s-event-definitions/
https://karun.in/blog/expected-threat.html

#9 Defensive Metrics [Transitions]

No matter how good a team are at maintaining possession, it is inevitable that you will lose the ball at some point. That’s okay, it happens, it’s not something you can prevent. What you can do something about is how you decide to react. This is where transitions come into play and happen so frequently that it’s important to get them right. Every time possession gets turned over, the attacking team need to change their mentality and positioning to reflect the fact that they are now defending. When considering defensive transitions, I will look at how quickly a team can make that change and what that change actually could be.

Usually an attacking team try to set up to maximise the space on the pitch, the players will be positioned high and wide to open up space between defenders. Whilst the defending team usually try to set up more compact, to deny space to the attacking teams and keep them away from key areas on the pitch such as near the penalty area and goal. Moving between these two mindsets efficiently throughout a match will determine games, the best teams are capable of seamlessly navigating between the two. Defensive transitions are moving from an attacking state to a defensive state. Moving from having possession with an intent to score a goal to not being in possession with an intent to get possession back and not concede.

Even in Figure 1 and Figure 2, Football Manager 2019 now acknowledges transitions when creating tactics to add to their realism.

When you lose possession, there are two main ways of trying to win the ball back. You can try to win the ball back immediately, this is commonly known as ‘counter pressing’ or ‘gegenpress’ popularised by Jurgen Klopp at Borussia Dortmund and now at Liverpool. If this isn’t an option then you will revert straight to a defensive set up that aims to deny space to the attacking team in key areas, this may be in your half or just approaching the penalty area. This is usually the default option for a team, especially for lesser teams against more threatening opponents. I’ll take a look at the benefits and drawbacks of each option and when each should be used.

When trying to win the ball back immediately, it usually requires a high burst of energy in the short amount of time after losing possession to swarm the opponent. An example of this is Barcelona under Pep Guardiola and their 6-second rule, where within 6 seconds after losing the ball the Barcelona players blitz the ball and opponent with the aim of forcing an error and retaining possession as quickly as possible.

The clear benefit to this style is that if it works, you minimise the amount of time that the opposition has possession and you maximise the time that you have possession since you win the ball back so quickly. Unfortunately, that’s under the condition that you do win the ball back. If you don’t win the ball back quickly, it’s hard to maintain such a high intensity of effort and pressure so you are forced into the second option and revert to a designated structure.

In that short space of time, the main aim is to win the ball back so defensive structure may be neglected. Again, if you can’t win the ball back quickly then it may take you longer to revert back to a designated structure and a team may capitalise on this extended transition period where players may be out of position. Due to the potential negligence in structure, this type of press is usually only used when losing possession in the opposition’s half. This gives you more time to revert back to a defensive structure if you fail to win the ball back.

When you fail to win the ball back immediately, or if you choose not to even try to, then you need to have a defensive structure that you move to every time your opponent has possession. The main aim is to not concede a goal and a mechanism to do that using a structure is to try to deny space to the opposition in key areas. What you define as a key area can depend on specific matches but generally a structure is constructed to deny space in your penalty area and anywhere within shooting range on goal. Depending on where you lose the ball can reduce the options that you have. Losing the ball in the opposition’s half allows you to try to deny ball progression into your own half before the opponent even get anywhere near your goal. Once they manage to get into your half you can then attempt to stop them progressing near to your penalty area. Whereas if you lose the ball in your own half then you need to immediately assume a structure that denies space near your penalty area.

Compared to turtling, a gegenpress will require certain type of players that are capable of frequent short high intense periods. Not every team has those players so that style isn’t even an option so some teams. The potential drawbacks of failing to win the ball back and neglecting defensive structure puts more emphasis on one on one defending and so is utilised more by teams that have a higher quality of individual players. When you turtle and drop deep to deny space, you are utilising the short spaces between players to cover and as a result you don’t need high quality individuals but those individuals to work as together. It’s a tactical decision whether or not to use the gegenpress and teams that are expected to win will use it as a way to gain an advantage with little risk.

@TLMAnalytics

Credit to Football Manager for acknowledging transitions in their tactics page, love the development

#7 Defensive Metrics [Decision Making]

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“If I have to make a tackle then I have already made a mistake.”

Paolo Maldini

It’s a famous quote I’m sure you’ll have heard, but you can hear the penny drop in every single person who hears it for the first time. One of the best defenders (if not the best) to have played football couldn’t be wrong could he. Yet defenders and defensive players are judged mainly on statistics such as number of tackles or blocks. Tackles and blocks are usually last-ditch attempts to prevent an opponent from progressing.

Defending is a constant ongoing process that is happening throughout a football match, no matter who has the ball or where the ball is on the pitch. As a collective team, and individually, every player is moving into positions that adhere to a defensive structure with an aim of conceding the least amount of goals possible. Each player will contribute to that by performing defensive actions, these are usually known as tackles or blocks. However, to perform a tackle or block first requires the opposition to have the ball in a potentially dangerous area, or rather first requires you to allow the opposition to have the ball in a potentially dangerous area. More importantly and less easy to quantify would be the actions and ability to prevent a forward getting the ball in dangerous areas in the first place.

It doesn’t seem a stretch to suggest that the something better than blocking every shot on goal is to prevent every shot being taken in the first place.

When a forward has the ball, they will have an aim in mind of what they want to achieve with their possession. There will be a hierarchy of aims ranging from scoring a goal down to retaining possession of the ball. Whilst a defender will also have an aim in mind when a forward has the ball. Their hierarchy of aims will be a version of the reverse of the forwards, ranging from not conceding a goal to winning the ball back. The immediate aim of both the forward and the defender will depend on factors such as location of the pitch, time of the game, game state and the perceived abilities of each player by each player.

For example, if the striker has the ball in the penalty area then their primary aim may be to take a shot to score a goal, whilst the defender’s primary aim may be to not concede a goal.

If the fullback had the ball in their own half then their primary aim probably won’t be to score a goal straight away, but rather progress the ball up the pitch either through midfield or down the line to the winger. If those two options are not available then they potentially need to regress their aim down to maintaining possession and recycle the ball back to goalkeeper or centre backs. In this case, the defender may be a striker or a winger who has closed the ball down, the defender’s primary aim here may be to prevent forward progression of the ball towards a more dangerous position.

Figure 1: Davies’ decision making options v Chelsea

These thought processes will be going back and forth between each player at all times throughout a match. Even whilst nowhere near the ball, these are things players need to consider at maybe a more minute level. Furthering the example above with the fullback and winger, the fullback’s aim is to ball progression and the winger’s aim is to prevent ball progression. If possible, the fullback would play the ball straight into the striker so that they could progress the ball up the pitch as far as possible as quickly as possible, however collectively the defence need to negate that as an option. Maybe the defending centre back is marking the striker tightly with the defending central midfielder also blocking off any direct pass, just enough so that the fullback doesn’t consider passing to the striker a viable option.

Figure 2: Chelsea unable to prevent Davies from progressing the ball

If the defending team sufficiently prevent efficient progress into dangerous areas of the pitch then their job is made much easier. As we can see in Figure 1 and Figure 2, Chelsea were unable to prevent ball progression, as a result they are left to defend a more dangerous situation and even resort to tackling or blocking (!).

The decisions that each player has, defender or forward, aren’t limited to just marking or blocking passing options and passing or shooting. Forwards may want to dribble past players, cross the ball from wide or even off the ball may make runs into space to receive the ball. These decisions of the forwards cause defenders to react respectively, how well they deal with the questions asked by the forwards depends on the abilities of the team and players in question.

It would be interesting to look at the decision making of defenders and forwards in different situations by counting the number of times or frequency of a decision overall and whether that depends on who they are facing or where they are on the pitch. A decision here for a forward would be a simple action such as attempt a shot, attempt a dribble, pass the ball up the line or retain possession. Whilst a defensive decision would depend on the decision of the forward, it would be interesting to see if players change their decisions significantly when playing against certain players. It could be a way to measure to what degree a defender can force a forward into uncomfortable positions and into making unfavourable decisions or decisions lower down on the forwards hierarchy of aims.

As always, any feedback or questions are welcome. These are primitive ideas and just looking to provoke thoughts of football analytics from a different perspective.

@TLMAnalytics

#6 Defensive Metrics [Optimal Positioning]

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Even though the most tracked part of a football match is where the ball is, the most interesting things often happen off the ball. The ball is only in play for 50-60 minutes of a 90-minute match, and each individual player is only on the ball a minimal part of that time. The majority of the game is played off the ball by all players, they need to move about the pitch in relation to their teammates, the opposition and the ball. A forward can move off the ball to find space between defenders to receive a pass, whilst the defenders need to keep an eye on the forwards and track these attempts.

For a specific player, at a given point of the game, there are locations on the pitch that would be considered worse positions to be in than others. For example, if the opposition had the ball on the edge of your penalty area, you would consider your central defender to be at a worse position if they were standing by the opposition’s corner flag than if they were marking the opposition’s forward. Since there is a concept of better or worse positions, that leads to the possibility of there being an optimal position for a specific player at a given point of the game. You could also think of it such as if you were to remove a single player from the game, where would you want to replace them in the game such that you couldn’t move them to a better place.

Several factors could affect the perception of a position at a given time being better or worse. These could be physical states of the game, such as locations of teammates, opposition or the ball. They could also be non-physical, such as score, the aim of the tactics or time on the clock. Considering where the ball is and who is in control of the ball will dictate the general area where your teammates and opposition will set up. Considering the tactics and formation that you and the opposition are looking to play will dictate the general areas of where individual players will set up.

Different tactical styles, scores and time remaining will affect what the aims of a team are. Some teams, such as Manchester City, want to control the largest surface area of the pitch possible. Control of the pitch can be determined by which team is likely to get possession of the ball if the ball was located in that area. Whilst other teams are aware that they can’t afford to try to control the largest surface area of the pitch, but rather look to control the areas of high interest such as around their own penalty area. Individual players need to position themselves with appropriate distances between each other to reflect their tactical style and goal. Certain distances between certain players would be better or worse than others, so again there must be an optimal distance with respect to tactical aim. When each player achieves this optimal distance, the collective team would appear to perform optimally.

A geometric way of viewing the areas of control on a pitch would be to look at Voronoi diagrams. 

Figure 1: Voronoi Diagram Red v Blue

If we look at Figure 1 with team red against team blue, each of the polygons surrounding each player would correspond to the areas on the pitch that they control. If the ball happened to be within their boundaries, they would most likely get to the ball first (considering each player has the same speed). This concept has been around for a while and has been made possible due to the technology available to football clubs, player tracking is everywhere nowadays and is crucial to understanding how your team is performing.

Voronoi diagrams can be used at a team level to understand structure and how well a team can transition between situations, but also is useful at the player level as you can identify which players find the most space or which players are the best at denying space.

In terms of quantifying better or worse positions of an individual player, the surface area of a player’s Voronoi region can be indicative of how well positioned they are. It is important to note that not all spaces on the football pitch are equal, controlling the areas closer to the goals area more beneficial than controlling the centre of the pitch. Perhaps a weighted surface area would be a better quantifier of control of the pitch and would be another contributor to identifying optimal positioning.

@TLMAnalytics


Special mention to below for their work already on Football Geometry and Voronoi Diagrams:
@Soccermatics –  https://medium.com/@Soccermatics/the-geometry-of-attacking-football-bee87e7a749
@UTVilla –  http://durtal.github.io/interactives/Football-Voronoi/

 #5 Defensive Metric Concepts [Expected Shot Block]

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There are emerging metrics in football such as Expected Goals, Key Passes, Progressive Passes and even now Expected Assists. These are all measuring single events in a football match and quantifying their utility or effectiveness with an indicator or a probability of happening. These are also all measurements of how effective a player is at executing actions whilst on the ball, particularly in offensive positions such as shooting or creating shots from passes. They give us a better idea for which players and teams are most (or least) effective at offensive events. The higher your Expected Goals and the more Key Passes you make, the more goals your team is likely to score. However, there aren’t similar metrics that measure defensive contribution. This may be due to the act of contributing to goal scoring being an objective decision, with each goal scored there is a single player who scored it and it’s easy to allocate contributions. With allocating defensive contribution, it is hard to quantify the presence of a non-event. It is hard to quantify how much of an effect a player or team has on the opposition not scoring. I think that if it’s possible to quantify a sensible defensive metric of any kind then it could be as useful as any of the offensive metrics above, I will use this series to brainstorm some ideas of such defensive metrics and how it would be possible to compute them.

Expected Shot Block:

Where better to start with defensive metrics than with the clearest act of denying a goal, the shot block. This concept is in direct competition to and is inspired by the concept of Expected Goals.

For Expected Goals, each shot is given a probability of being a goal based on historical shots of a similar type. For example, a shot that is taken with the head from a cross may be given 0.1 xG whereas a shot from a counter attack inside the 6-yard box may be given 0.5 xG. It suggests that the shot from a counter is 5 times more likely to go in than the headed effort. These may not be realistic numbers but the concept stands. Some shots are more likely to go in than others depending on a number of factors including where on the pitch the shot was taken, what play led to the shot, what the shot is taken with and game state of the match.

The concept of the Expected Shot Block would be to calculate the probability that the shot is blocked by a defender. This would require more information than just the event data of the shot, it would require the knowledge of the presence of a defender and how likely it is that a defender makes a block in a similar situation based on historically similar shots. The time this decision is made would be at point of contact of the shot. Based on shots from the past, you can categorise them into similar categories as that of Expected Goals but with the added factor of the presence of a defender or defenders between the ball location and the goal. The ball location and the two posts of the goal create a triangle and if there are any defenders in this area then the shot would be identified as having the potential to be blocked. The presence of the goalkeeper is expected and since we are looking at shots being blocked not saved then the goalkeeper’s location can be acknowledged but not required for calculation. The location of the goalkeeper may alter the shot direction of the attacker so may affect shot blocking numbers.

Expected Shot Block - FM

Furthering the concept of an Expected Shot Block would be to calculate the percentage of the goal that is open to the shot at the point of contact. When identifying if a shot has the potential to be blocked, you can calculate the percentage of the goal that is available to be shot at where the defenders wouldn’t make a block. This calculation could be done in either 2D or 3D. You can assume an average area for the defender’s body and block out the area of the goal that the defender is in front of. In 2D this would be less accurate than in 3D since it would be assumed that the defender can block a shot of any height. Whereas in 3D, you could create silhouettes of the defender’s limbs and create a more accurate percentage that way.

There are some problems with this concept but I think it has potential. You need more than just event information, you also need player location data which is harder to get. It also assumes that the shot is a direct hit straight at goal, whereas many players attempt to bend and curl the ball around defenders. Just because a defender is in the way of the goal, doesn’t guarantee a blocked shot in that location. There are many times where a shot goes through a defender’s legs or just past a limb, defenders and players aren’t perfect.

It’s hard to quantify defensive actions and shot blocking seemed to have the most relevant as it’s related to the current set of offensive metrics, it’s not perfect. If anyone has any thoughts or comments regarding other issues I may have missed, please do let me know!

@TLMAnalytics

#3 Are Man City Better Without The Ball? – Defensive Analysis

@TLMAnalytics

In this piece I will take a look at what makes Manchester City such a good defensive team by looking at the types of recoveries that they make. Comparing games that they have dominated and those they have ‘struggled’ in (used very loosely) suggests there’s a reason why they have looked less clinical in some games, and can identify a potential chink in the City armour.

We all know that City are one of the greatest teams at keeping hold of possession. Pep Guardiola has brought with him and adapted his style of play from Barcelona and Bayern Munich with great success. They are beautiful to watch, passing the ball around the pitch with such patience, precision and ease that it makes you think you could do it watching from the couch. What’s not immediately obvious is the defensive prowess of teams under Guardiola and how they manage this, despite not even training tackling (!).

“I am not a coach for the tackles so I don’t train the tackles.” – Guardiola, Dec 2016

To score a goal, you need to have the ball, and when playing against Manchester City you don’t get the ball for long. This means that you need to make every time you do have the ball count, if you are wasteful then you might not see it again for a while. The problem is that as good as they are in possession, once Manchester City lose the ball they are arguably even better, making it extremely difficult for opposition teams to take advantage.

They have played 14 games this season and only twice have they made less recoveries than their opponents. Considering City are so good with the ball, you may hope that they are at their weakest when they don’t have it. This shows that they are just as good, if not better than most at getting the ball back.

Not only are City at least as good as everyone else at recovering the ball in general, in terms of where they recover the ball, they are by far better than most. Out of 724 total recoveries made so far this season, 231 were made in the opponent’s half (~32%). Whereas only 112/628 recoveries were conceded in City’s half (~18%). This means that City are recovering the ball higher up the pitch more often than against them, which is important since there is less distance to goal and usually fewer defenders the higher up the pitch you win the ball back.  Even more astounding is the fact that the minimum number of times City have recovered a ball in the opponent’s half is 11, which was away to Arsenal in the first game of the season. Every game they make at least 11 recoveries in the opponent’s half, the most so far was against Fulham at home (shock) where they made 25 recoveries in Fulham’s half.

image2

Whilst only on four occasions have their opponents recovered possession in the City half more than 11 times. These games were the three away games against fellow top 6 members (Arsenal, Liverpool and Spurs) and their only defeat, at home to Lyon in the opening Champions League game.

Out of the 14 games City have played, these four were among those where they looked the least clinical version of themselves. They beat Arsenal 2-0 however arguably should have won by more. The 0-0 draw against Liverpool looked like a game with two teams who didn’t want to lose cancelling each other out, hardly any chances were created in that game. They won 1-0 against Spurs in another game where arguably City should’ve score more. In their 1-2 loss to Lyon with Guardiola in the stands, they met a clinical Lyon side and couldn’t create a big chance all game. In each of these games City didn’t look at their best, and in each of them, their opponents recovered the ball in City’s half as much as City recovered the ball in their half (63 v 54). In the other 10 games, City recovered the ball in the opposition half 168 times and conceded recoveries only 58. That’s only four more than the four least clinical games City had.

Not only does recovering the ball high up the pitch prevent the opposition from getting anywhere near your goal and therefore no chance to score, it also instantly puts you on the front foot and creates better scoring opportunities for your own team. We know that City are such a good team with the ball, however this suggests that many of their great chances probably also come from winning the ball back from teams high up the pitch. It may seem obvious and easier said than done, but if you are able to prevent City from doing this to you or if you are able to recover the ball high up the pitch against them then that looks the best way to disrupt them. Limiting the number of times they recover the ball in your half gives you the chance to move the ball further towards their goal and prevents them from exploiting your defensive transitions.

The players with the most recoveries per start for Manchester City seem to be the players who have played the most this season. This suggests that it’s something Guardiola keeps an eye on and favours in his players. I have counted the number of times a player has made 5+ recoveries in a game compared to how many starts that player has had. The top 3 are Mendy (8 times/9 starts), Fernandinho (12/14) and Laporte (10/14). The games which Mendy missed, Delph (3/4) and Zinchenko (1/1) covered for him and appears to be a recovery gold mine at City’s left side.

Credit to @StatsZone for the graphics and recovery numbers.

@TLMAnalytics