Why Possession Value Is Bollocks

Expected goals. Expected assists. Expected passes.

How likely is a shot from here to be a goal? How likely will a pass from here to there turn into a goal if the guy receiving the ball shoots? How likely will a pass from here to there be successful?

All models have flaws and these guys are no exception. But they all have one thing in common.

They pretty much work as intended.

Why?

Because they are fairly simple by design.

They are fairly simple by design because they are singular actions.

On top of this, 99 times out of 100 we can be sure of a player’s intention when making the play or in the case of expected assists, infer it afterwards because a shot happened regardless of the initial intention.

We know what we are trying to measure here, and its not difficult to do if we have the data.

The trouble is, these models are a bit boring now.

The new guy on the block in Analytics Town is the Possession Value type metric. Our chums at Opta define the idea thusly:

– OptaPro’s Possession Value (PV) framework establishes the probability of a team scoring from an individual possession.

– The framework assigns credit to individual players based on positive and negative contributions, covering key on-the-ball events.

Does it work?

When looking at preliminary results, we noticed a major negative influence on the scores for players who are often involved in attacking plays.

We believe it is crucial to assign blame and/or credit only where it’s due. Therefore, in our framework, the punishment for the loss of value of the possession is capped at 0.025 (the average value of a possession).

That’ll be a no.

I am not against the idea of expanding analytics into more exciting, more complex football actions. But let’s build on solid foundations.

Let’s look at the idea itself again:

– OptaPro’s Possession Value (PV) framework establishes the probability of a team scoring from an individual possession.

Is this fairly simple by design? Well, the actions are no longer singular, multiple actions take place in a possession.

Where does a possession begin and end? Up for debate. Where does the following Everton possession end from the initial kick off v Brighton yesterday?

Is it when Djibril Sidibe’s launch down the touchline gets cut out by Dan Burn? If so, does Alex Iwobi’s touch that finds Bernard start a new possession? Or does no team really have the ball under control here and it’s a possession for neither?

My view is that the whole passage is Everton’s possession until about 18-19 seconds when Brighton (to me) gain full control of the ball back. I’m sure some of you will disagree. I know for sure that the main data suppliers all do.

Immediately we have problems with definitions of what a possession actually is.

– OptaPro’s Possession Value (PV) framework establishes the probability of a team scoring from an individual possession.

Is the intent of every possession to score a goal? If not, then why are we measuring all possessions against it? How many players in the clip are thinking: “If I do this, we’re more likely to score.”?

What’s the actual thought process from the Everton players there? Is it more like: “Ooh fuck, I’m under pressure here, let’s just keep it and if it’s too risky we’ll send it long, so if we lose the ball further up the pitch, there’s less danger”?

If it is then what’s the point of measuring it against the probability of scoring?

At each stage of a possession, the intent in moving the ball is different. It can move from retention, to advancement, to retreat to deliberately giving the ball away in a less dangerous area.

Possessions are a building block to get from A to B to C to D etc on the pitch and then create openings for goal opportunity.

So measure them in separate blocks.

Measure attackers v each other in the final third.

Measure centre backs on ball advancement to the next thirds of the pitch.

Measure midfielders on how brave they are in not going backwards all the time to alleviate pressure.

A goal may be the ultimate team goal, but it is not the ultimate goal of every player. Stop creating models that pretend it is. Possessing one has no value.

 

 

 

 

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