Football and the (mis)use of statistics

I have found a second reason to get interested in football (*): it can help teach students basic lessons about probabilities and more generally the use and misuse of statistics. Here is an example taken from Nate Silver's website, fivethirtyeight.com, (my emphasis in the text):

(*) As in this previous post, it does nor require watching the games, thankfully ;-)

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Reep’s research was quite groundbreaking for its time, even if it was fatally flawed. The Wing Commander gathered data on how often a given number of successful passes were strung together, and how frequently goals resulted from those sequences, broken down by length. Reep determined that a team’s probability of retaining possession dropped precipitously with each consecutive pass attempt, and that most goals were scored on possessions of fewer than three passes — often originating from quick counterattacks.

In Reep’s mind, this meant teams should abandon trying to control possession and maneuvering through the defense with endless passing. Instead, they should focus on getting the ball downfield in as few movements as possible on offense, and applying pressure on defense to generate opportunistic counter-rushes. The numbers seemed to suggest that the long game was the most efficient tactic for soccer success.

But subsequent analysis has discredited this way of thinking. Reep’s mistake was to fixate on the percentage of goals generated by passing sequences of various lengths. Instead, he should have flipped things around, focusing on the probability that a given sequence would produce a goal. Yes, a large proportion of goals are generated on short possessions, but soccer is also fundamentally a game of short possessions and frequent turnovers. If you account for how often each sequence-length occurs during the flow of play, of course more goals are going to come off of smaller sequences — after all, they’re easily the most common type of sequence. But that doesn’t mean a small sequence has a higher probability of leading to a goal.

To the contrary, a team’s probability of scoring goes up as it strings together more successful passes. The implication of this statistical about-face is that maintaining possession is important in soccer.² There’s a good relationship³ between a team’s time spent in control of the ball and its ability to generate shots on target, which in turn is hugely predictive of a team’s scoring rate and, consequently, its placement in the league table. While there’s less rhyme or reason to the rate at which teams convert those scoring chances into goals, modern analysis has ascertained that possession plays a big role in creating offensive opportunities, and that effective short passing —fueled largely by having pass targets move to soft spots in the defense before ever receiving the ball — is strongly associated with building and maintaining possession.

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