Explicação 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:

• Location of shooter: How far was it from the goal and at what angle on the pitch?
• Body part: Was it a header or off the shooter's foot?
• Type of pass: Was it from a through ball, cross, set piece, etc?
• Type of attack: Was it from an established possession? Was it off a rebound? Did the defense have time to get in position? Did it follow a dribble?

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:

• Comparing xG to actual goals scored can indicate a player's shooting ability or luck. A player who consistently scores more goals than their total xG probably has an above average shooting/finishing ability.
• A team's xG difference (xG minus xG allowed) can indicate how a team should be performing. A negative goal difference but a positive xG difference might indicate a team has experienced poor luck or has below average finishing ability.
• xG can be used to assess a team's abilities in various situations, such as open play, from a free kick, corner kick, etc. For example, a team that has allowed more goals from free kicks than their xGA from free kicks is probably below average at defending these set pieces.
• A team's xGA (xG allowed) can indicate a team's ability to prevent scoring chances. A team that limits their opponent's shots and more importantly, limits their ability to take high probability shots will have a lower xGA.

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:

• Behrens' first shot with the goalkeeper in his way = .41 xG
• Behrens' second shot with the goalkeeper out of position but a defender in the way = .47 xG
• Zreľák's shot with an open net = .79 xG

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:

• As mentioned above, the penalty kick attempt = .79 xG
• The second shot after the rebound, from 2 yards and with the goalkeeper unrecovered from the save = .92 xG

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.

O que é o Post-Shot xG (PSxG)?

O xG normal, ou o que pode ser considerado o "Pre-Shot xG", é calculado considerando todos os chutes, no momento do chute, sem saber a qualidade da tentativa de chute. Isso não inclui somente os chutes que acertam o alvo, mas também os que são desviados para longe dele. O Post-Shot xG é calculado após o chute ser dado, após sabermos se o chute acertou o alvo, levando em conta a qualidade do chute. Assim como com o xG, o PSxG é oferecido pela Opta e é melhor explicado aqui.

Todos os chutes que forem para longe do gol terão PSxG igual a zero, já que há 0% de chance deste levar a um gol.

Ao avaliar a capacidade de defesa de um goleiro, só queremos incluir os chutes que tenham acertado o gol, já que estes são os chutes onde o goleiro pode ter algum impacto. Assim, usamos o PSxG para estimar a qualidade dos chutes que eles enfrentaram.

O que são xA (assistência esperada) e xAG (gol assistido esperado)? Qual a diferença entre eles?

xA, ou assistência esperada, é a probabilidade de que determinado passe realizado se torne uma assistência de gol. Esta estatística desenvolvida pela Opta atribui uma probabilidade a todos os passes com base no tipo de passe, na localização em campo, na fase do jogo e na distância percorrida. Os jogadores recebem pontos xAs por cada passe realizado, independentemente de um chute a gol ter ocorrido ou não.

Para isolar apenas os pontos xGs em passes de assistência para um chute a gol, há os Gols Assistidos Esperados (xAG). Isso indica a capacidade de um jogador de criar chances de gol sem ter que depender do efetivo resultado do chute ou da sorte/habilidade do jogador que chutou. O jogador só recebe pontos xAG quando faz um chute a gol após um passe realizado.

Usamos pontos xG+xAG para contribuições de gols, já que as contribuições de gols dos jogadores geralmente são Gols + Assistências e isso corresponde melhor a esse padrão.

Até outubro de 2022, usávamos pontos xA para indicar gols assistidos esperados (agora pontos xAG). Quando mudamos nosso provedor de dados para a Opta, eles forneceram sua versão do ponto xA descrita acima. Fizemos a mudança de nome para ponto xAG. Opta: o que são assistências esperadas.

Onde encontrar o xG

O xG de Time, xG contra e o diferencial xG podem ser encontrados nas tabelas de liga, tal como essa:

xG de jogador, npxG e o xA podem ser encontrados nas páginas dos times, tal como essa:

Os gols previstos também podem ser encontrados em várias páginas diferentes, como as estatísticas de jogador de liga, relatórios de partida, páginas de jogador e registros de partidas de jogador.

Competições FBref com dados xG

• Copa do Mundo Feminina FIFA (2019 to 2023)
• Copa do Mundo FIFA (2018 to 2022)
• Liga de Campeãs Femininas da UEFA (2021-2022 to 2024-2025)
• Liga dos Campeões da UEFA (2017-2018 to 2024-2025)
• Liga Europa da UEFA (2017-2018 to 2024-2025)
• UEFA Euro Feminina (2022)
• American National Women's Soccer League (2019 to 2025)
• Brazilian Série A (2019 to 2024)
• English Championship (2018-2019 to 2024-2025)
• English Premier League (2017-2018 to 2024-2025)
• English Women's Super League (2018-2019 to 2024-2025)
• French Ligue 1 (2017-2018 to 2024-2025)
• French Première Ligue (2021-2022 to 2024-2025)
• German Bundesliga (2017-2018 to 2024-2025)
• German Frauen-Bundesliga (2022-2023 to 2024-2025)
• Italian Serie A (2020-2021 to 2024-2025)
• Italian Serie A (2017-2018 to 2024-2025)
• Italian Serie B (2018-2019 to 2024-2025)
• Major League Soccer (2018 to 2025)
• Spanish La Liga (2017-2018 to 2024-2025)
• Spanish Liga F (2022-2023 to 2024-2025)