The metric xG is predominantly used for strikers to measure how good they are. xA is used to credit creative players with the quality of their key passes. But what about the pass leading to the assist, the pre-assist? And the pre-pre-assist? And so on, do they not deserve credit for xG?
By looking at the possession chains and the xG of the shot that results from them, we can give credit to the players for their attacking contributions they make outside of the shots and key passes. The xG chain (xGC) is a straightforward metric that can be used to reveal the contributions of players involved in the earlier stages of attacking moves, such as (creative) centre-backs and playmakers. It gives analysists a simple way to highlight attacking contributions in pass maps.
How it is calculated:
Find all of the possessions a player is involved in.
Find all of the shots within those possessions.
Sum the xG of those shots.
Assign that sum to the player.
(Repeat for every player)
Any of action on the ball counts whether it is the first pass in a 50-pass build-up or the player who takes the shot, the credit is the same.
As with most metrics, they are put as a per 90, xGC/90. Usually, the players who have a high xGC/90 are going to be forward as they are more likely to be involved in plays which lead to a shot. However, Philipp Lahm was seventh in the 2016/17 for xGC/90 out of players who had played 600+ minutes. Lahm’s value of 1.38 xGC/90, 0.03 ahead of teammate Robert Lewandowski and 0.35 behind Lionel Messi’s 1.73 xGC/90 which was the highest in Europe’s top 5 leagues.
However, the values are still being dominated by xG from shots and xA from assists. There is a metric to give more credit for build-up play and some unexpected players are able to shine through. xGBuild-up is the same as xGC but the shots and key passes are removed from the possession chains to highlight a player’s involvement within build-up play. It does involve players who take the shot or create the key pass if they are involved in earlier parts of the play.
Now that there is a rough metric for attacking contribution, analysts can look at teams and see how balanced they are. Teams who are balanced tend to be more successful than those who are (over) reliant on a few players.
To see this, a Gini coefficient would have to be calculated for sides over their open play xGC/90 values (for players who have played 600+ minutes). The lower the value, the more balanced their attack is – a value of 1.0 would mean a team generates all of its xG through one player whilst a value of 0.0 means a team involves all of its players in every attack.
However, it is important to remember these numbers don’t necessarily tell the full story. In 2015/16, Leicester City won the league with a coefficient of 0.28 and RasenBallsport Leipzig (RB Leipzig) finished second the following campaign despite being in the bottom half for their offensive balance.
Although it can’t necessarily predict how well a team will perform, it is important in terms of recruitment and squad planning. It can act as a warning sign if a team is over-reliant on players in attack.
To conclude, xG chain is a flexible and simple metric. It can be used to expose aspects of attacking play that other statistics miss. It should not be used in rating a player’s offensive skill, it is evolutionary. It in fact builds on existing models and is another tool analysts can use.