xPG is Back, Baby!: Putting Possession Metrics to the (Correlation) Test

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xPG is Back, Baby!: Putting Possession Metrics to the (Correlation) Test

Last year, Cheuk Hei Ho, Eliot McKinley, and I collaborated on a soccer metric called Expected Possession Goals (xPG). xPG is a possession-based non-shot expected goals metric designed to measure the value of possessions whether they result in a shot or not. You can read more about it in these initial articles here on American Soccer Analysis. Later last season we extended xPG into multiple variants, including measuring possession risk and mistakes, but so far the initial xPG, now called Chance xPG, has been the most interesting. We even created a Twitter account called @GameFlowxPG which measures match momentum. It has been pretty popular (I’m a little salty since it usually has more followers than I do).

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Our Favorite ASA Articles of 2018

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Our Favorite ASA Articles of 2018

During a recent American Soccer Analysis shareholders meeting in the penthouse suite of the swanky hotel we built in Minecraft (it’s our Slack channel), we discussed our favorite ASA articles of the past year. Because it is the season of listicles and we relish every chance to talk about ourselves, we decided to put them all together in one official post. Also, our site traffic is essentially zero at this time of year, so it seemed like an easy way for us to remember where we put them.

It was a great year for MLS (though perhaps not American soccer overall) and the most successful in our five year life as a website. We added interactive tables, introduced xPG, rebooted the podcast (new episode coming out soon! …probably), and added a lot of great new writers to our existing ranks of stale old writers. They’re not all represented in the list below, but special shout out to our weekly contributors who put together content every week - Little Things (@harrisonhamm21), Lowered Expectations (@harrison_crow), Expected Narratives (@ahandleforian), and Setting the Table (@ericwsoccer) - showed us the individual plays each week that made up the whole of the MLS season. We’d also like to extend a special thank you to Neil Greenberg of the Washington Post, for including us as a part of the WaPo’s incredible World Cup coverage.

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Expected Possession Goals GameFlow: Taking a Ride Into the Danger Zone

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Expected Possession Goals GameFlow: Taking a Ride Into the Danger Zone

A few weeks ago, we introduced Expected Possession Goals (xPG) GameFlow, a visualization of the momentum of a soccer match from kickoff to final whistle of each game. xPG GameFlow uses the accumulation of Chance xPG to measure the strength of an opportunity for a team to get a shot. The higher the Chance xPG differential between the teams, the longer the bar for that minute for the team with the higher amount. Quite often goals are scored when the momentum bars on the xPG GameFlow chart are at their longest.

Many people have asked us, “what is the difference between xPG and xG?” or “how does xPG translate to xG or to goals?” To aid xPG GameFlow in answering questions such as “which team had the better chances?” and “when should a team have scored?”, we introduced a couple improvements after the first week of tweeting MLS game charts on @GameFlowxPG. I wanted to provide more context for these improvements and dive deeper into them.

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Ebb-and-flow: Using xPG to capture soccer’s momentum swings

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Ebb-and-flow: Using xPG to capture soccer’s momentum swings

Recently we introduced Expected Possession Goals (xPG) as an experimental metric. In our latest article, we introduced four uses of xPG. Like any good experiment, things are subject to change as you take input. To help xPG be a little easier to consume, we’re updating some of the terms to be more understandable:

Chance xPG (formerly called Positive xPG) is the total value of a possession based on weighted values gained from actions such as ball-winning actions, passes, dribbles and shots. It is assigned to all the players involved in a possession. A player or team with high Chance xPG is getting the ball into higher opportunity areas for a shot. Chance xPG is expressed in a positive value.

Shot xPG (formerly called Successful xPG) is the total Chance xPG earned by possessions ending in a shot. Players and teams with a high Shot xPG relative to Chance xPG are ones which are good at turning chances into shots. Easy, right?

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Chris Armas’ transformation of the New York Red Bulls

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Chris Armas’ transformation of the New York Red Bulls

Coaching the New York Red Bulls must be a dream for most managers in North America's soccer circle, but Chris Armas also has had one of the toughest tasks in MLS. A mid-season takeover is never easy, let alone the takeover of a contender from the legendary Jesse Marsch. The Red Bulls organization may have boasted that they focus on the same pressing style starting from the academy, but everyone has their own unique ideas they want to implement. Armas is treading a fine line: he is introducing new elements while also keeping what was working for Marsch. The Red Bulls are still playing a similar style of soccer, so it appears Armas has been making quantitative, rather than qualitative, changes. Deciphering those changes will require some analytics techniques.

I first look at how New York has fared under the two managers using different variants of Expected Possession Goal (xPG). I recommend you read that full article, but in short it’s a score that measures the risks a team bears vs the rewards it creates. In short, Negative xPG measures the risks a team bears, while Mistake xPG measures the amount of turnovers a team commits from those risks.

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The Next Level of xG: Expected Possession Goals

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The Next Level of xG: Expected Possession Goals

Using xPG variants to assess risk-and-reward of the game

We introduced Expected Possession Goals (xPG) in two recent articles. xPG groups and rates the outcome of a possession and began from an idea that every action in the possession connects to create a shot. Here, we’re introducing new xPG variants, extensions to the original xPG definition to assess the risks and rewards inherent in a soccer possession.

xPG rates a group of uninterrupted events - or when an interruption lasts fewer than two seconds - based on where the ball travels. It assumes the purpose of the possession is to move the ball within shooting distance.

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