Thursday, 17 January 2019

A Non Shot Expected Goals Look at the UCL Group Stages.

The last post looked at quantifying the increased contribution made by players attempting progressive passes based on the improvement in non shot expected goals via completing a pass and the likelihood that an average passer is able to successfully make such a pass.

We've been building non shot xG models for a few years, so lets take a look at how possession & passing ability can be redefined in terms of non shot xG from this season's UCL group games.

Once you have a NS xG framework you can look at the risk/reward of every attempted pass by quantifying the improvement in NS xG should the pass be completed.

This can be further combined with the likelihood a pass is completed against the risk of losing the initial NS xG you owned and handing NS xG to the opposition should they take possession.

To simplify the post, I'll just look at the reward side of the bargain and aggregate the expected value of a completion in NSxG units for all progressive passes attempted by the 32 UCL group teams and compare that value to the actual value of the completions they made.

This will quantify how often a side had possession in a dangerous area of the field and if, through better passers and/or receivers they outperformed an average passing team.

We'll also take a look at the value of passes allowed into dangerous areas and whether a side managed to reduce that value by making it difficult for opponents to complete passes compared to an average defence.

The defensive side of the ball is often ignored or described entirely in terms of completed actions, such as tackles or interceptions, with little context.

The "Attacking Reward from Progressive Passes NSxG" column is the model's average expectation that a progressive pass results in a possession somewhere on the field.

Playing a forward pass out of defence to the centre circle is very likely to be completed, but the value of the possession in the centre circle won't be that large.

Playing the ball into the opponent's penalty area, dependent upon the origin of the pass, won't be as easy to complete, but will result in a relatively large NS xG value if it is.

Overall, if an average team was willing and able to attempt the pass attempts of Real Madrid in the group phase, they would expect to accrue a cumulative NSxG of 74.2 NSxG over the six games.

Real actual gained 77.9 NSxG.

So they made lots of dangerous pass attempts (although they did also recycle the ball backwards) and over performed the average model by 4% based on actual completions.

Porto was one of the better defences. They allowed side's to make progressive passes worth a model value of 39.4 NS xG and restricted the completions to further depress the actual value to 36 NS xG over the six games.

The best offensive and defensive performers, in terms of NS xG accrued or allowed, along with above average efficiencies are shown in blue, underperformers in red.

Attack and defensive numbers are correlated, particularly from a possession standpoint. As Swansea showed possession can be a purely defensive strategy. So it makes sense to look at the attacking and defensive differentials, along with the performance of the 32 teams in the group phase.

Real Madrid had a net positive NSxG differential of +44.2 in topping group G and Crvena Zvezda a whopping -57.8 in propping up group C.

Real got the ball often into dangerous positions with above average efficiency and restricted the ability of opponents to do the same at league average efficiency.

This is a step towards quantifying progressive passes, rather than simply counting final third completions etc. It unsurprisingly tallies with actual performance and provides a framework to produce possession chain based evaluations of past and future games that isn't entirely reliant upon a shot based approach.

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