The Cult of xG
Lots of talk on the Sounders interwebz and the xG statistic. What is it and what does it mean?
Very briefly, xG tries to measure shot quality in a game. Over the course of the season xG attempts to reflect how sustainable a team's offensive production is.
Now, to determine xG you must make rather large assumptions about how likely it is that a goal is scored from a given chance. It does this by ignoring the strengths or weaknesses of the player taking the shot, and the relative strengths and weaknesses of the goalie attempting to save that chance.
For example, a chance in the penalty area thats falls to Ruidiaz is not the same as an identical chance that falls to the feet of Yeimar. xG, however treats these opportunities the same. Likewise, it assumes the goalkeepers are all created equal, and we know that's just not the case.
Let's just say it measures what SHOULD happen (historically speaking) in a game but not what actually does happen.
Even the people who really like using xG will admit that the stat can be misleading when analyzing an individual game. Nevertheless, it's always shown with the other stats such as shots and possession at half-time and is frequently relied on in post-game analysis anyway.
Proponents claim that xG is more reliable over a period of games or a whole season. However, whether or not a team underperforms or over-performs its xG does not reflect where they will finish in the league table at the end of the year, but a high xG is generally good. How this is any different than the team that scores the most and lets in the fewest goals usually wins I'm really not sure.
Still awake? Still with me? Now lets get to the drama!
People online, as they inevitably do, get really mad when discussing this stat. Strong opinions all around. It really riles people up more than numbers and stats usually do.
On Sounders Twitter, the xG cultists are trying really hard to convince the non-believers that the Sounders are actually playing really well and that the xG and advanced stats reflect this positive state of affairs.
The response can be generally summarized as: then why is it that when we watch the games, the Sounders don't look so good? And how does one get excited by 2 wins out of the last 7 MLS games and dropping out of the Open Cup in the same time period? And for Tahoma's sake, Why must we keep losing to Portland?
Cultists: But our xG and the stats show a strong team, don't worry, everything will come out good and the Sounders will start scoring, just as it will inevitably start raining again and we will be on our way to the playoffs and glory!
Doubters: But could you tell us why the Sounders have such good xG but don't seem to be scoring? Could you offer us some analysis that goes beyond - well it's a bit of a mystery actually, and mostly bad luck?
*Silence*
The crux of the argument appears to me to be about whether the Sounders are playing well or not. xG says yes, 2 wins out of 7 says no. Depends on which argument persuades you I suppose. Time will reveal all.
What is agreed upon, generally speaking, is that currently this team is not playing at the level that anyone expects them to. Whether you understand that through the lens of xG or just by watching and saying, yeah these performances don't look so great, most fans agree that currently the team has the potential for better soccer, better performances, and especially better finishing.
If stats help you to understand what is going on and helps bring you to a deeper understanding of the game, then that's wonderful. But ultimately statistics reflect reality, not the other way around, and when the statistics don't align with what people are seeing, there will be disagreements about what is happening on the pitch.
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