I’ve been looking at defensive statistics for individual players for several weeks now and, while I’m still very much at a basic level, there are some things that I think I’ve noticed that I feel I should put down on virtual paper.
Defensive stats on an individual player level are quite difficult because you’re trying to measure something invisible, essentially how many goals does this person prevent out of the number of chances that they have to prevent goals. Not only is the thing we’re measuring invisible, but many of the indicators are hard to quantify. Off the ball movement is huge, as it can put the defender in the right place to shut down an attack before it properly develops.
However, off the ball movement data isn’t collected at the moment in football, and even if it was I imagine it would be hugely difficult to use. You wouldn’t just need the defender in question’s off-the-ball heatmap, you’d presumably need the positional data of the player on the ball at any given time, as well as various attackers and probably the rest of the defensive line too. That’s about six players’ positional data you would need to have and be able to analyse just for a single defender.
Decision making would also be difficult to judge, I imagine, from statistics alone. Firstly, because it would probably involve deciphering the positional data discussed above. Secondly, though, there would have to be a way to accurately apportion blame for an opposition attack without being too distracted by the success of that attack. If a defender makes several poor decisions in a match but, luckily, they go unpunished that doesn’t necessarily make them better than a player who makes one mistake that is punished by a goal. Alternatively, though, the one mistake that led to an opposition goal could have been a HUGE error in judgement, whereas the several instances of poor judgement by the first defender may have just been small ones.
There is also the issue of how to adjust a defender’s defensive score for how dominant his own team is. Obviously, a defender who only stops a few attacks but they’re the only attacks they have to deal with has performed better than a defender who only stops a few attacks while his team was on the back foot for the whole match.
I know a few people are using possession to adjust for dominance however a team may have a lot of possession but do little in the way of attack with it, which would unfairly harm the opposition defenders’ scores (as their team had little possession meaning their scores are reduced, however they may still have had very little to do). This is why I use shots on target and shots blocked as well as possession, as I feel this gives a better indication of how dominant, in an attacking sense, teams are. I don’t think it’s perfect, but I do think it’s better than only using possession.
A potential problem is that I work out the player’s score for each match and then take an average of those, rather than taking an average of every stat from the whole season together, if that makes sense. I’ve not thought about this too much, and I’m it’s a while since I did any proper maths, so I’m not exactly sure whether taking a lump end-of-season stat would create different figures than stats for each match, but I guess it might be a possibility.
A similar-ish problem occurs when players play in varying positions throughout the season. This is an advantage of doing things match by match because it means, in my case for example, I only input the data when players played at centre-back. If I did it match by match indiscriminate of their primary position for that match then the data could be skewed (for example David Luiz played about half his time in central midfield last season, so if I took every minute he played on the pitch and put that on a central defender radar chart template then it wouldn’t be an accurate indication of his centre-back performance). This is a potential issue because 1) playing in position ‘x’ might naturally give you better or worse stats in certain areas than playing in position ‘y’ (as a crude example, you’ll score more goals up front than in goal) and 2) because players might perform to different standards or play in different ways depending on what position they are in.
Those are all my thoughts for the moment, and I’d genuinely love to hear what people think. Either comment below or get me on Twitter @ETNAR_uk