Collated by Alex Bartiromo

Here at American Soccer Analysis we try to publish cutting edge articles on soccer that don’t rely on narrative, anecdote, or lazy comparison, preferring instead to rely on data, experimentation, and a ruthless questioning of all of our own previously held beliefs. As you might surmise from this somewhat pompous description, doing that is hard, and our writers have more opinions and insights than we could possibly publish. That is why we are coming out with a new series, the ASA roundtable, a weekly discussion between our contributors on questions facing the soccer and soccer analytics communities today. We look forward to your feedback, and if you want to submit a question just tweet it to @analysisevolved or email us!

Alex Bartiromo

Welcome to the ASA Roundtable, folks!

Given the proliferation of analytics in the soccer community, we've gotten much better at examining the various contributions players make to a team, especially when it comes to goal scoring and the actions that lead up to it. However, one area that is still little understood is defensive contributions. How can analytics help us understand defending in modern game? What are the next steps that need to be taken to improve our understanding in this area? What are some analytical concepts you think more MLS teams could take into account when game planning/building rosters that they aren't currently (if any)?

Carlon Carpenter

To start off, the defensive ability of a player or a team is hard to judge purely based off of simple metrics that are used for attacking players: defensive players can’t be judged off of simple accumulation of tackles, headers, etc. because the number of actions is largely skewed by team strength. Thankfully, tracking data is coming into play more and more. Through GPS data and other similar tools, we will likely be able to generate a more holistic viewpoint of what makes good defensive players.

Cheuk Hei Ho

You can look at "space control" by drawing a circle around the player based on how far they can reach and measure pressure by offensive event success, but this time looking at what defenders are nearby.

Harrison Crow

To Carlon's point—and Cheuk's too—it's not just what makes a player good defensively but what makes a team good defensively. I think of Real Salt Lake, which, honestly, isn't a team that stands out in terms of defensive ball winning ability. But they've been extremely good at limiting shots over the summer and from what I can tell, a lot of that has to do more with combined team work than it does any one single player and their ability. Knowing the tactical awareness and marking ability of a group of players is just as important as understanding a player’s ball winning potential.

Cheuk mentions space control and it's true. It's not just pressuring the ball but marking and making sure that when a player receives a pass that they don't have the ability to turn or move the ball in a way that could compromise your team’s shape or create angles for passes to break lines for runners.

Cheuk Hei Ho

It seems to me defense is always about shape, being compact, etc. You could do that with tracking data now.

But even assuming we have all the tracking data that we want, event data is still gonna be needed because you still need to use the offense to infer the effectiveness of the defense.

Carlon Carpenter

Combining the event data with tracking data can give a pretty good outline of the area a defensive player “dominates”. The more space a defender has control over, the better player.

Harrison Crow

Pushing the conversation towards roster construction, something to look at is the performance of a player in their team’s context. When we talk about defensive events and how a player performs, a lot of times we fail to consider how much, or how little, his team is responsible for possession. Possessions and pass attempts, or movements in general, often change the tone of how we talk about tackles, interceptions, and blocked shots.

For example, a team that plays in a low block is probably going to gather more opportunities to block a shot. So saying this player is "great" because he simply has a lot of blocked shots doesn't account for the fact that while he's on the field, his team may play with fewer possessions of the ball, giving him more opportunities to block a shot.

Carlon Carpenter

To continue Harrison's point on roster construction: we’re at a point now that teams would be remiss to not search for players who play in similar systems and styles to “filter out” those who numbers wouldn’t match the same output (in the same areas too!) of those in other systems. For example: if you’re a counter attacking team, you look for defenders in other counter attacking teams.

At the end of the day, players are largely products of their system.

Eliot McKinley

We all agree that tracking data is going to be a game changer for defensive metrics, but how long do you think it is going to take before something useable will be widely available?

Carlon Carpenter

Well, do you mean publicly? For pro clubs, etc.? Because Catapult data (those sports bras players wear), is already doing a lot with this.

Eliot McKinley

I’m thinking of this quote from the Athletic about hockey tracking data.

“We’re getting player tracking data in the NHL over the 2019-20 season, the first public burst of it ever, and it’s going to give us amazing, interesting, helpful statistics that hockey fans decades ago couldn’t have even fathomed. But in the process of figuring out what’s important, and how to use a lot of this information, there’s going to be a lot of shit out there. I mean flat-out shit.

is GPS tracking good enough? Do you need optical tracking?”

I know people like STATS and SportLogiq are already looking at these things

as well as whatever Barcelona and Liverpool are doing.

Drew Olsen

Even MLS clubs are doing it. In fact Sean Davis talked about this very thing on our last podcast.

He did mention that the sports bras weren't the most comfortable though.