Illustrazione degli xG

What is xG?

Very simply, xG (or expected goals) is the probability that a shot will result in a goal based on the characteristics of that shot and the events leading up to it. Some of these characteristics/variables include:

Every shot is compared to thousands of shots with similar characteristics to determine the probability that this shot will result in a goal. That probability is the expected goal total. An xG of 0 is a certain miss, while an xG of 1 is a certain goal. An xG of .5 would indicate that if identical shots were attempted 10 times, 5 would be expected to result in a goal.

There are a number of xG models that use similar techniques and variables, which attempt to reach the same conclusion. The model that FBref uses is provided by Opta. Opta's xG model includes a number of factors above just factors such as the location and angle. Their model also accounts for the clarity of the shooter's path to the goal, the amount of pressure the shooter is under from defensive players, the position of the goalkeeper, and more. That means that their xG model factors in the defense and goalkeeping when determining the odds of the shot reaching the goal.

Take this Diego Jota goal vs Southampton for example. The shot was taken directly in front of the goal from very close range. It's a very good chance. Using an older model that accounts for location, angle, pass type, and such, it would have a 0.68 xG. However, Opta's model also accounts for the fact that the goalkeeper is out of position and there's no defender in the way, which boosts the xG of this shot even higher, to 0.90.

xG does not take into account the quality of player(s) involved in a particular play. It is an estimate of how the average player or team would perform in a similar situation.

How xG is used

xG has many uses. Some examples are:

Penalty Kicks

Each penalty kick is worth .79 xG since all penalty kicks share the same characteristics. Comparing a player's goals from penalty kicks to their penalty kick xG can indicate a player's penalty kicking ability. Likewise, we can do the same for goalkeepers in these situations.

FBref's xG totals include penalty kicks unless otherwise noted. For xG excluding PK, we recommend using npxG (non-penalty expected goals).

How we calculate xG totals for a single offensive possession

In some cases, a player or team's xG totals do not equal the sum of their shots. For instance, a team may attempt multiple shots in a single possession, but it is likely that these shots are contingent upon the outcome of the previous shot(s).

Take for example, this match between Schalke 04 and Nürnberg:

View Match Highlights on YouTube

In the 78th minute, Nürnberg attempted three shots which ultimately led to a goal. Hanno Behrens attempts a shot that is saved, but he is able to take a second shot as the ball is deflected off the defender. The second shot goes off the woodwork, which allows Adam Zreľák to easily tap it in. According to Opta's expected goals model:

The sum of these three shots is 1.67 expected goals, even though it is impossible to score more than one goal in a single move. To solve this problem, we find the probability that the defending team does not allow a goal in this possession. In this case, the calculation is:

(1 - .41) x (1 - .47) x (1 - .79) = .0657 or a 6.57% probability that Schalke does not allow a goal.

To find Nürnberg's xG, we simply subtract that probability from 1:

1 - .0657 = .9343 xG

In other words, we estimate that an average team in a similar situation would be expected to score a goal 93.43% of the time.

We use a similar method when calculating xG for individual players. Adam Zreľák receives .79 xG from his single shot while Hanno Behrens receives:

1 - (1 - .41) x (1 - .47) = .6873 xG

This shows why a team or player's total xG may not equal the sum of the xG from their shots and why a team's total xG may not equal the sum of the xG from their players.

Possessions that include a penalty kick

Similarly, we include shots taken from a rebound after a penalty kick with xG from penalty kicks. Take this Marco Reus penalty kick for example:

Since the second shot is a result of the first, we use the same probabilistic method in the previous example. Rather than a total 1.71 xG (.79 + .92), the calculation is:

1 - (1 - .79) * (1 - .92) = .9832 expected goals

However, since the second shot is also considered to be a part of the penalty kick xG, Reus gets 0 npxG (non-penalty expected goals) on this play.

Note: We treat corner kicks and free kicks as a new possession, not a continuation of the previous possession, but are continuing to study the issue.

Che cos'è il Post-Shot xG (PSxG)?

Il Post-Shot xG, o quello che può essere considerato "Pre-Shot xG", viene calcolato considerando tutti i tiri al momento del tiro senza conoscere la qualità del tentativo di tiro. Non comprende solo i tiri a segno, ma anche quelli deviati o fuori bersaglio. Il Post-Shot xG viene calcolato dopo che il tiro è stato effettuato, una volta che si sa che il tiro è in target, tenendo conto della qualità del tiro. Come per l'xG, il PSxG è fornito da Opta ed è ulteriormente spiegato qui.

Tutti i tiri fuori bersaglio avranno un PSxG pari a zero poiché c'è lo 0% di possibilità che questa traiettoria porti a un gol.

Quando si valuta l'abilità del portiere di fermare i tiri, vogliamo includere solo i tiri che vanno a bersaglio poiché questi sono quelli su cui il portiere può avere un impatto. Pertanto, utilizziamo PSxG per stimare la qualità degli scatti che hanno affrontato.

Che cosa indicano xA (assist previsti) e xAG (goal con assist previsti)? In che cosa differiscono?

xA (assist previsti) fa riferimento alla probabilità che un passaggio completato si trasformi in un assist. Questa statistica sviluppata da Opta assegna una probabilità a tutti i passaggi in base al tipo di passaggio, alla posizione in campo, alla fase di gioco e alla distanza percorsa. I giocatori ricevono xA per ogni passaggio completato, indipendentemente dal fatto che un tiro sia stato eseguito o meno.

A separare gli xG sui passaggi che costituiscono assist a un tiro ci sono i goal con assist previsti (xAG). Ciò indica la capacità di un giocatore di creare occasioni da goal senza dover fare affidamento sull'esito effettivo del tiro o sulla fortuna/abilità di chi calcia. I giocatori ricevono xAG solo quando viene effettuato un tiro dopo un passaggio completato.

Utilizziamo xG+xAG per i contributi ai goal, poiché i contributi ai goal dei giocatori sono in genere goal + assist e ciò rispecchia questo standard al meglio.

Prima di ottobre 2022, con xA indicavamo i goal con assist previsti (ora xAG). Quando siamo passati dal nostro fornitore di dati a Opta, questi ha fornito la sua versione di xA descritta sopra. Abbiamo modificato il nome in xAG. Opta: cosa sono gli assist previsti.

Dove si trovano gli xG?

Gli xG di squadra, gli xG contro e il differenziale degli xG sono reperibili nelle classifiche dei campionati, come questa:

Premier League Table
Pos. Squadra PG V N P Rf Rs DR Pt xG xGA xGD
1Manchester City3832249523+729884.324.7+59.6
2Liverpool3830718922+679773.728.8+44.9
3Chelsea3821986339+247258.636.4+22.2
4Tottenham38232136739+287154.947.1+7.8
5Arsenal38217107351+227060.154.2+5.8
6Manchester Utd38199106554+116661.450.6+10.8
7Wolves38169134746+15752.142.1+10.1
8Everton38159145446+85449.745.7+4.0
9Leicester City38157165148+35252.443.7+8.7
10West Ham38157165255-35247.661.9-14.3
11Watford38148165259-75048.259.2-11.0
12Crystal Palace38147175153-24947.650.1-2.5
13Newcastle Utd38129174248-64539.153.6-14.5
14Bournemouth38136195670-144553.357.2-3.9
15Burnley38117204568-234044.462.1-17.7
16Southampton38912174565-203946.955.1-8.2
17Brighton3899203560-253635.359.1-23.8
18Cardiff City38104243469-353442.461.5-19.1
19Fulham3875263481-472641.368.2-26.8
20Huddersfield3837282276-541628.860.9-32.2

Gli xG, gli npxG e gli xA dei giocatori sono reperibili sulle pagine relative alle squadre, come questa:

Statistiche ordinarie 2018-2019 Manchester City: Premier League Table
Tempo di gioco Rendimento Prestazione prevista Progressione Per 90 minuti
Giocatore Nazione Ruolo Età PG Tit Min 90 min Reti Assist G+A R - Rig Rigori Rig T Amm. Esp. xG npxG xAG npxG+xAG PrgC PrgP PrgR Reti Assist R + A R - Rig R + A - Rig xG xAG xG+xAG npxG npxG+xAG
Ederson Moraesbr BRAPor2438383.42038.0011000200.00.00.10,10300,000,030,030,000,030,000,000,000,000,00
Aymeric Laportees ESPDif2435343.05734.0336300303.03.00.83,89429490,090,090,180,090,180,090,020,110,090,11
Bernardo Silvapt PORCen,Att2336312.85431.77714700307.47.47.815,21521562770,220,220,440,220,440,230,250,480,230,48
Raheem Sterlingeng ENGAtt2334312.77130.81792617003013.713.79.623,3155874360,550,290,840,550,840,440,310,760,440,76
Sergio Agüeroar ARGAtt3033312.45927.32182919224018.116.55.021,581762530,770,291,060,700,990,660,180,850,600,79
Kyle Walkereng ENGDif2833302.77930.9112100300.80.81.92,783220920,030,030,060,030,060,030,060,090,030,09
David Silvaes ESPCen3233282.40126.76814600207.87.88.516,31182702220,220,300,520,220,520,290,320,610,290,61
Fernandinhobr BRACen3329272.37726.4134100501.61.63.04,558236290,040,110,150,040,150,060,110,170,060,17
İlkay Gündoğande GERCen2731232.13723.7639600304.14.14.38,482205910,250,130,380,250,380,170,180,350,170,35
Leroy Sanéde GERAtt2231211.86720.71010201000106.76.77.414,184673410,480,480,960,480,960,320,360,680,320,68
John Stoneseng ENGDif2424201.76419.6000000100.30.30.20,64411850,000,000,000,000,000,020,010,030,020,03
Riyad Mahrezdz ALGAtt,Cen2727141.34314.97411701005.54.74.69,387731910,470,270,740,470,740,370,310,680,320,62
Nicolás Otamendiar ARGDif3018141.23613.7000000101.31.30.21,5279230,000,000,000,000,000,100,010,110,100,11
Oleksandr Zinčenkoua UKRDif2114141.15112.8033000100.20.21.51,74795940,000,230,230,000,230,010,120,130,010,13
Vincent Kompanybe BELDif3217131.22413.6101100600.30.30.00,3178330,070,000,070,070,070,020,000,020,020,02
Kevin De Bruynebe BELCen27191197510.8224200201.41.45.77,050109880,180,180,370,180,370,130,520,650,130,65
Benjamin Mendyfr FRADif24101090010.0055000100.20.21.61,84870590,000,500,500,000,500,020,160,180,020,18
Danilo Luiz da Silvabr BRADif271198079.0101100100.40.40.20,62077330,110,000,110,110,110,050,020,070,050,07
Gabriel Jesusbr BRAAtt212981.03611.573106111011.210.52.312,735211280,610,260,870,520,780,970,201,170,911,11
Fabian Delpheng ENGDif281187258.1011000110.10.10.30,42059230,000,120,120,000,120,010,040,060,010,06
Phil Fodeneng ENGCen181333353.7101100002.12.10.93,02318350,270,000,270,270,270,570,230,800,570,80
Philippe Sandlernl NEDDif2100
Arijanet Muricxk KVXPor1900
Claudio Bravocl CHIPor3500
Totale di squadra26.7384183.42038.09171162883444184.381.365.5146,71325242924122,391,874,262,324,182,221,723,942,143,86
Totale di squadra26.7384183.42038.09171162883444184.381.365.5146,71325242924122,391,874,262,324,182,221,723,942,143,86

I dati relativi ai gol previsti sono reperibili anche su una serie di pagine diverse, quali statistiche dei giocatori del campionato, resoconti delle partite, pagine dei giocatori e registri degli incontri dei giocatori.

Competizioni in FBref contenenti i dati sugli xG