Ask anyone the name of the first human to ever walk on the moon. There’s a high chance they’ll know the answer to that. Ask them who the second human was. The answer to this question? Not so much. This analogy holds water even when it comes to team sports.
In football, for example, attackers that finish the move are the ones that are glorified, more often than not. Not the one that made the defence-splitting pass, not even the guy with the assist, but the guy who tapped the ball into an empty net. Come to think of it, the players that make the final pass before the goal is scored are recognised too, to an extent. But what about those players who were vital in the build-up to the goal?
Often overlooked, these players don’t receive the same accolades their attacking counterparts receive, but there’s probably a good reason for it. How do you quantify their contributions to a goal? It’s not as black or white as goal or no goal, or, assist or no assist.
Sure, as humans, by virtue of the complex brain we possess, we can attribute some value to it. But you can’t possibly quantify their contribution consistently, repeatedly, and in an unbiased fashion for every pass completed in every game played.
This is where xGBuildup comes in handy. Although it has its own inherent flaws, it does a good job in identifying and highlighting the players that are important to a team’s build-up play. xGBuildup came into existence due to xG or expected goals. (I highly recommend watching the embedded video below)
xG is so important today that it has become part and parcel of the way players and clubs alike, are incentivised. As mentioned in the video below (view from 15:15 to 16:29), players and coaches are now getting incentivised on their expected goal tally, instead of their actual goal tally, by a few forward-thinking clubs.
This way “Clubs are rewarding the process, rather than the outcome” according to Omar Chaudhuri, the Head of Football Intelligence at 21st Club.
So now (hopefully!), you understand what xG is and why it’s important, but what about xGBuildup. How is that calculated, you ask? It’s actually really intuitive.
Let me explain using an example. Let’s take the goal Christian Eriksen scored against Manchester United on 31st Jan 2018 (Video for the goal starts at 0:48 in the tweet below). It was the 3rd fastest goal in PL history (the ball hit the back of the net 10.54 seconds after kickoff).
This is how the ball progressed. Mousa Dembélé kicked off the proceedings and passed the ball back to Vertonghen who optimistically lobbed the ball towards the United penalty box. Harry Kane headed the ball into Dele Alli’s path who backheeled it towards an onrushing Eriksen, who slotted home the ball from just outside the 6-yard box.
In other words, the ball progressed like this –
Dembele => Vertonghen => Kane =>Alli (assist) => Eriksen (goal).
Let’s assume that the expected probability of the goal being scored under those particular circumstances (i.e. from that distance, angle etc) by Eriksen is 0.4 or in other words, the xG of that goal is 0.4.
Formerly, only Eriksen would have been rewarded with an xG of 0.4. But now, xGBuildup can reward the players who were involved in the build-up to this goal (or for that matter even in the build-up for any passage of play that just ended up in a shot)
In this passage of play, every player involved other than the player who provides the assist and the player who scored, is rewarded. That is, in this case, Dembele, Vertonghen and Kane are all rewarded with a 0.4 addition in their xGBuildup tally. According to StatsBomb, xGBuildup can be calculated in the following manner:
1. Find all the possessions each player is involved in.
2. Find all the shots within those possessions.
3. Sum the xG of the shots
4. Assign that sum to each player, however involved they were.
Basically, as illustrated in the example of Eriksen’s goal above, xGBuildup is found for every shot taken by a team. Therefore, if 6 players are involved in a passage of play that ends up in a shot, the 4 players that are involved in the build-up before the assist and the goal, will get rewarded with the xG of that shot.
Hopefully, this example helps in understanding xGBuildup and its value as a statistical metric. (This statistical metric can be used by teams to smother their opponent’s attacks by marking their most important player, during their build-up play, out of the game. This can have a lasting effect on the game, as Jorginho himself has admitted more than once)
But using the raw xGBuildup metric without normalising it over a 90 minute period does not make any sense. This is because xGBuildup value for a player who has been on the pitch for a longer time will tend to be greater. So, to level the playing field, we can divide a player’s “xGBuildup” value by his “Total Minutes Played” value to get xGBuildup/Total Mins. After this is done, we can multiply this value by 90 to get xGBuildup/90 or xGBuildup Per 90 minutes
xGBuildup/90 = (xGBuildup/Total Minutes Played) * 90
18 out of the 20 clubs before the start of this game-week had played 21 games in the EPL. That’s a maximum total of 1890 minutes (21*90, excluding extra time). To make sure we don’t encounter any outliers or players who weren’t regularly involved, any player who appeared for his club for less than 800 minutes (out of a possible 1890) was excluded from this list. This means that all the players in this list have at least 800 minutes under their belt this season
So here it is (the teams appear in alphabetical order). Listing every Premier League team’s most important player during their build-up of play in this season
#1 Arsenal – Granit Xhaka – xGBuildup Per 90: 0.446
Love him or loathe him, Granit Xhaka is an important peg in this slightly disfigured Arsenal side. Not exactly a fan favourite, Xhaka has been an important player under all of the managers he’s played during his stint at Arsenal. He has been remarkably injury-free, a trait that Arsenal midfielder’s don’t usually possess, having started 109 out of a possible 135 games for the club since he signed for them.
Xhaka comes in at number 13 in the PL list for highest xGBuildup but out of the Top 13, 9 are Manchester City players. So if Manchester City players were not considered in this list, Xhaka would come 4th in the PL. Arsenal will have their work cut out if they were to replace him if he does leave in the summer because of how important he has become in Arsenal’s build-up play, off late.