Possession is probably one of the football statistics which is cited most often. It has gained even more media attention as we have seen larger divergence in possession across teams. Frequently game approaches of teams are broadly categorized between possession-oriented and counter-attacking: think about Barcelona, Bayern Munich, Manchester City vs Chelsea, Borussia Dortmund and Liverpool. At first glance the possession statistic seems to be fairly trivial; who controls most of the ball during the 90 minutes? There are however some details that can have quite an impact on the final number. The simplest approach is to approximate possession with touches. This can however favour teams that try to play short and controlled passes and prefer a slow build-up play. On the other extreme you could try to assess possession for every second of the game, even interpret a uncontrolled clearance as giving up possession. The OptaPro Blog has written a very good article on this exact topic. In thi...