When you are watching a football match every once in a while you'll see flashed up on the screen a statistic showing how much "ball possession" each team has had. Opta explains how this statistic is computed:
[T]here are several data providers out there in the UK and across the world monitoring games, from TV companies themselves for live games, to specialists like Opta.
Each has their own method of working out possession. Some use calculations based on the data, but most use a “chess clock” approach where each team has a button which is hit when they are in possession. Some do this in the broadcast truck, others have analysts who call it out and inputters who hit the buttons.
Opta used this method originally, but the problem we found with a chess clock approach for time is that you are reliant on the person logging the data remembering to hit the button and the person doing it usually has other tasks to perform and other data to log.
Missing a couple of switches obviously skews the possession figures and it’s impossible to go back and change it. It may not sound much but one minute where the clock is wrong can affect the possession figures by two to three percentage points.
Opta now record possession in a football match by means of an automated calculation based on the number of passes that a team has in a game. We have two analysts, each monitoring one of the teams and they log each event in a game, totalling between 1600 and 2000 events per match.
Each of these events has a timecode plus an xy co-ordinate and the collection system is rigorously monitored by our team of checkers.
During the game, the passes for each team are totalled up and then each team’s total is divided by the game total to produce a percentage figure which shows the percentage of the game that each team has accrued in possession of the ball.
For Opta "ball possession" means percentage of completed passes, and is not a measure of time, though Opta does claim that the two are very closely related.
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