Do Newton's Laws apply to football pundits?
27 April 2014
My kids are currently addicted to a CBBC programme called "Absolute Genius", and it's some of the best children's TV I've ever seen.  Each week the presenters Dick & Dom focus on a famous scientist, inventor or engineer in history, and with the help of experts and experiments they do an extremely good job of explaining why folk such as Alan Turing were such heroes.  Johnny Ball would be proud of this stuff.
Last week's episode was about Isaac Newton, and my eight year old now has a good grasp of what gravity and g-forces are all about.
Apparently, though, the show didn't mention momentum - which is a shame, because the world of sports punditry might have learned something.
There's been plenty of "momentum" on the airwaves this week, in reference to Liverpool Football Club who, prior to their match against Chelsea, had won a record-breaking 11 consecutive Premiership matches.  I lost count of how many times I heard about Liverpool having the 'momentum' to win the league.  (Other teams, meanwhile, are being urged to regain some momentum, Aston Villa and Norwich in particular.)
But does 'momentum' mean anything when applied to sport?  Is there even evidence that it exists?
Before the Chelsea game, I provocatively tweeted that a coin that comes up Heads 11 times in a row has no momentum.  The next toss is as likely to be a tail as a head. My implication was that football results are a bit like tossing a coin, and maybe winning eleven matches in a row has little bearing on the result of the next match.
On the other hand, if teams genuinely do have 'momentum' that would imply that a team that has won X games in a row is going to require a considerable force to stop this trend from continuing.
Of course the huge difference between a coin and a football team is that a coin has no memory. Football teams involve a bunch of people who will have been cheered up if they won their previous match and demoralised if they didn't. What happens in this match is bound to be affected by what happened in previous matches: unlike coins, the probabilities are not independent.  That's what makes sport more interesting than watching coins being flipped.
The question is how much this psychological effect is measurable above the extreme random effects of kicking a ball at goal.  A team might have ten shots on target in a match and on average only score from one of them.  But it's often a question of inches, and those shots on target might easily have ended up in two goals (and a win) or no goals (and defeat).
And in any case, there are situations where 'momentum' might make it HARDER for a team to win their next match.  At Anfield, it's clear that Chelsea adopted the tactics of trying to disrupt Liverpool's flow, partly by being more defensive than usual.  It possibly reduced Chelsea's chance of winning, but it reduced Liverpool's chance of winning too.  Indeed the BBC commentary just fifteen minutes into the game declared that Chelsea had managed to stifle Liverpool's momentum.  That didn't take much force, did it?
And sometimes momentum might count against a team.  They might get over-confident and careless.  Or they might suddenly get spooked at the prospect of winning their first trophy in years. Momentum becomes an albatross.
If this 'momentum' thing in football (and other sport) really exists, there should be statistical evidence for it.  Finding that evidence is quite a statistical challenge.  If a team wins 11 matches on the trot, the chances are high it was already a very good team, so it was already more likely to win the twelfth game than lose it.  If it were possible to model all football teams as biased coins, then Chelsea/Liverpool/Man City might be coins which come up heads 70% of the time, while Cardiff does so 30% of the time.  A change of manager/tactics/players will affect the coin, so these have to be thought of as coins with constantly changing biases.
I'm sure there is something in the notion of a team having momentum.  After all, in a blog back in January I argued that after the first two Ashes Tests in the winter, Australia became so confident and England so demoralised that each match became increasingly one-sided after that.  But sporting history is also full of examples of shock wins by underdogs who defied momentum entirely (not least Sunderland a couple of weeks ago against Chelsea).
And remember, there's another old adage in football.  You're only as good as your last game. And now they have lost to Chelsea, what has happened to Liverpool's momentum? Can't have it both ways.