Wednesday, 7 October 2015


Once more the topic of PDO in football has risen to be debated among the analytics Twitterati; opinion has been kicking around for a few days and the Analytics FC podcast chatted around the topic too. I got a mention as someone who uses PDO in analysis, which amused me as generally, but not exclusively, I consciously try to avoid using it in writing. 

No matter, i'm well aware of the issues with the metric but slightly mystified as to the strength of anti-opinion it still generates.  Nobody is claiming PDO as a top line metric, as James Grayson has shown regularly, it does not habitually repeat and regresses over time.  What it does do is give a useful one hit overview of a metrics that a team has little to no functional control over.  Precisely because it oscillates around a fixed point gives it a degree of simple understanding.

As I write West Ham have a PDO of 122 which is high and incredibly unlikely to sustain.  Similarly Southampton and Liverpool have PDOs at 78 and 81, which are equally low and equally unlikely to sustain.  What does this tell us? Via one number we have an instant method of cutting through the overlying narrative that a team is good or bad; we can see whether a team is running hot or cold.  For this reason, some of the strengths of the metric as an informer lie around its ability to define reasonably short periods of time. 

At its extremes we find teams that are interesting and it offers us a route in for for further analysis.  Having highlighted West Ham via their extreme PDO, I can then quickly zoom in on the factors that are contributing.  Their save percentage is slightly high, but not too extreme (74%) but the rate in which their on target shots are being converted is huge (49%) and simply will not continue at that level long term.  Then we can look at shot levels and be less than impressed, or their wider all shot conversion and ponder that either their finishing talent is akin to Barcelona or it's a freak run. 

Liverpool are dying at both ends, Southampton's problems are all in the save percentage.  We spot this with a one hit metric, then we look deeper for further issues.  Are these teams spending a lot of time in deficit or ahead? Are they crippled by injuries to key players? What is going on?

Nobody conducts in depth analysis using PDO alone, like any metric, in isolation it has limited use.  Its components are far more useful as part of a chain of metrics that inform on different aspects of a team's qualities.  Chief among the chain will likely be a version of a shot metric be it simple or an iteration of expected goals and more and more we seem to be moving towards ideas around zones. 

A by-product of all this is as complexity increases and standardisation remains tantalisingly out of reach, there is still value in metrics derived from actual events.  With reticence still prominent in many aspects of football-- from clubs to media to fans-- there is a small duty among analytics proponents to maintain a degree of accessibility and understanding.  With many people focusing on a move towards club work, it is worth noting that recommendations for the forthcoming Optapro forum have placed an emphasis on relevance and applicability and we've certainly not reached a point where rejection of simply derived metrics benefits the whole.

And how do you replace a simple concept?

A kind of plus-minus expected goal figure is hindered by the aforementioned issue of non-standardisation and complexity of derivation and even in usage largely highlights the very same teams as a basic PDO calculation. Anyway the factors involved here are not fixed measurements of a team's quality.  Like a recipe, relevant ingredients come in different quantities yet combined can create the right mix.  For too long too much focus has been put on issues with this metric, be it the mysterious name or non-intuitive meshing of non-related aspects, and missed its simple strength; as a simple temperature gauge for a team's performance.

That's all it really does, and all it needs to.

Time to move on.

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