I'm wondering how it is posssible to calculate the amount of ball possession that each of the two teams has in a game of football. We often see a statistic such as 65% - 35% for example. Is it the amount of time that the ball is in one half of the pitch compared to time in the other? Or, does someone keep a stopwatch running clicking it on and off based on some judgement as to which team has possession? If so, what happens during fast paced possession changes and times when no-one has possession? Must be quite subjective and imprecise-- unless there is a simple rule of thumb....?