Tuesday, 2 July 2019

Quantifying the Value of Every Pass

I've written about passing models over the last couple of years and posted passing maps for individual players and teams recently. So here's a quick overview the passing model upon which those maps are based, how it was developed and how they might be useful.

The model is derived from location and time stamped Opta data for every pass attempt. The model has been build in conjunction with Infogol, but as yet it isn't part of the data available on the Infogol app.

I was keen to use familiar units for the passing model, therefore all values for successful or unsuccessful passes are expressed in expected goals.

I've purposely avoided such things as distance gained, as this often leads to arbitrary definitions for "key passes".

It also breaks down entirely when you approach the penalty area, not only in terms of scaling, but also assigning value to a backward pass that actually adds value to a side if it is completed. (Think pull backs from the goal line, a "progressive" pass can easily go backwards).

The baseline values are the likelihood that possession at any position on the field will end with a goal and is taken from historical data.

Therefore, if passing from one point to another improves the likelihood of a goal, the successful pass is quantified as the change in this likelihood.

Because the unit of measurement is how likely historically, possession is to turn into a goal, it doesn't require a goal attempt to ultimately be made at the culmination of the move.

This is a huge advantage over passing models that are based solely around attempts being taken because every pass attempt is counted (a player is not reliant on success or failure further down the passing chain).

It also makes the calculation of speed of attack much more relevant to the actual threat present (advancing the ball ten yards in a couple of seconds from the final third causes much more of a threat than advancing the ball 20 yards in half the time from your own penalty area).

Finally, because the units relate to aggregated historical outcomes of possessions, we can quickly give a value to any point of the field, which is not the case if the point value is based on the expected goals of an actual goal attempt from that position.

And so because a goal attempt isn't required the units are designated as non shot expected goals to differentiate them from shot based xG.

To keep things simple, the following non shot xG passing maps omit other actions, such as carries or dribbles, take no account for time of possession or any likely passing skill differential.

The following maps simply record any successful, "progressive" pass (by which I mean any pass that advanced the likelihood of a team scoring) made by a player during the last Premier League season.

The maps are simply conditional formatting in excel, on a 10X10 grid, overlaid with a pitch. 10X10 is used for the convenience of Opta's x,y pitch locations which run from 0-100, lengthwise and widthwise.

The darker the conditional formatting the more NS xG has been gained from a successful pass from that location. Either by small gains, but large passing volume, large gains and fewer passing volume or a combination of the two.

It's easy to show the passing distribution through other plots.

Here's England's newly capped Declan Rice's successful progressive NS xG gains for WHU in 2018/19.

This represents the starting point of every successful pass.

The plot is best used in conjunction with video analysis, but you can quickly see that Rice's sphere of influence is concentrated broadly in front of the back four and across the line, but he also delivers an impressive range of threatening passing options mid way inside the opposition half and just leftfield.

The next thing we'd like to know is where these passes end up, so the following plot illustrates where on the field this improvement in NS xG production from Rice via his passes is distributed and received by a team mate.

Overall Rice's progressive passes are received around 10% further upfield than their points of origin. He spreads the ball wide, as noted by the darker areas on the flanks either side of halfway and towards the final third. And he finds a team mate on the edge of the box, but doesn't appear to be a predominate passer of the ball into the box (particularly if we strip away set plays).

Rice appears to be an active and productive passer over around three quarters of the playing area, but may not be fully appreciated because he rarely plays a pass that may be considered an assist.

By contrast, here's a much more attacking NS xG passing profile from Manchester City's midfielder, David Silva. A darling of the highlights reel.

 Unlike Rice, Silva rarely ventures into his own half to begin build up play. The starting point for his progressive passing is a hot spot just outside the left edge of the opposition penalty area, although he does occasionally drift to the opposite side of the box.

The end point of his passing is again strongly centred around the left side of the field, but deep into the opposition box. He sticks rigorously to the left sided channels and relatively shuns pass attempts to the right side of the box from his team's perspective.

Finally, for now, we can also show where a player is showing up as the recipient of a progressive pass.

Once again his fondness for linking up with a team mate in the dangerous left hand side of the area is shown, firstly by the darker formatted green area just inside the left flank of the area on the plot and secondly in an actual example from a game.

This just scratches the surface of how these plots, maps and quantified valuing of passes can be useful in assessing a side, or a player. It is particularly welcome because it removes the highlight reel aspect that blights player assessment (particularly on youtube immediately following a transfer). We can see from the heat maps if creative passing into particular areas of the field is a largely consistent player trait....or if the exceptional pass, that perhaps resulted in a goal was a once in a lifetime fluke.

This post has concentrated on progressive, NS xG gaining successful passes, but it can also be applied to unsuccessful attempts to measure risk reward, the probability of a pass being completed can also be added and we can also look at ball retention plots to see which players excel at retaining the ball for others to make the decisive progressive deliveries.

Rice and Silva obviously play different midfield roles in widely differing teams, but their respective importance and discipline in playing a role it those two systems becomes much more apparent once we look at their passing contribution as a whole.