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やっぱり、計算間違いだったので
スポナビの記事は消しました。
http://www.eplindex.com/21707/possession-tenths-law-stats-analysis.html
面白かったんだけどねぇ
Is possession always nine tenths of low?
って記事。
ただし「Shot/poss」って単語紛らわしい。
本当に1シュートあたりのポゼッションだったようだ。
計算が合わなかったから記事ごと消した。
効率の良さを言いたかったみたいだが。

なお、ポゼッションに関しての記事といえば
optaがポゼッションに関する記事を出してきましたね
http://www.optasportspro.com/en/about/optapro-blog/posts/2012/possession(s).aspx
その中でのポゼッション定義もなるほどと。
"Half of the people can be part right all of the time,
Some of the people can be all right part of the time.
But all the people can't be all right all of the time"
(a) possession starts upon a player controlling the ball
This should be pretty straightforward. Specifically, winning a tackle or header does not, of itself, indicate the start of a possession. Only the first controlled action thereafter does.

(a) possession ends when a player no longer has - or tries to have - control of the ball.
This is where it becomes a bit more open to debate. While it is straightforward to define a possession ending when a player's pass is intercepted or they dribble out of play there are some grey areas.