Sunday, November 20, 2011

Momentum in Sports

The Freakonomics blog makes a point that I think is wrong:
The best place to start is with a famous (for academia) paper from several years ago, called “The Hot Hand in Basketball: On the Misperception of Random Sequences.” As you can glean from that snazzy subtitle, the authors come down against momentum, arguing that a “hot streak” is really just a random sequence that we misperceive to be more meaningful than it is.

Ever try flipping a coin 100 times? You’ll be surprised at how many long, unbroken sequences of heads or tails you get. It’s easy to mistake that for a pattern, suggesting some kind of meaning or momentum, but it’s really just a pure illustration of randomness itself. The fact is that if you get 10 heads in a row, the next flip is no more likely to be heads (or tails, for that matter).

And so it is, for the most part, with hot hands and hot streaks and hot quarterbacks. In our Momentum video, you’ll hear Toby Moscowitz, the academic co-author of Scorecasting, discuss how pretty much everyone in football believes in momentum. But, having looked at a lot of NFL data, Moscowitz reaches a sobering conclusion: “There is a much stronger belief in momentum than is warranted by what we see in the data.”
* * *

Consider one example in our video, the Buffalo Bills’ redonkulous 32-point comeback against the Houston Oilers in 1993. As Chris “Mad Dog” Russo puts it: “You’re gonna tell me momentum had nothing to do with that game?!”

Okay, Chris, I’ll take a shot at telling you exactly that. You know why we’re still talking about that game? Because it was a massive anomaly – the kind of comeback that almost never happens. It was so rare that our brains have an easy time recalling it. (We do this with all anomalies – dramatic plane crashes, mass murders, and so on.) And when we recall something so easily, we tend to believe it’s far more common that it actually is.

The truth is that you’re bound to get a wild 32-point-comeback once in a while, just as you’re bound to get a streak of 10 or 12 heads too.

Here's the thing: a 32-point comeback might indeed be so rare that it fits within a statistically normal distribution as several standard deviations above the mean. This does NOT mean, however, that a 32-point comeback was itself a matter of random chance -- the 32-point comeback happened because of how a bunch of human beings performed on a given day, and their performance was not random at all. Their performance was affected non-randomly by their preparation and skills, their coaching, their choices of plays, and their confidence level (the latter of which would be dramatically affected if either team started to think that the "momentum" was heading in a particular direction).

Try an analogy: A 7-foot-tall man is a rarity, and if human height falls into a normal distribution, someone might make the following claim, akin to the dismissal of sports momentum: "This 7-foot-tall man's height might seem to have sprung from some genetic factor, but in fact, you find 7-foot-tall men in nature only as often as would be expected by chance. Therefore his height is just a matter of random chance, not genetics."

Well, the fact that this particular guy got the genes to be 7 feet tall might be random chance from the point of view of a statistician looking at all of humanity, but that in no way proves that his height was unrelated to genes. Similarly, the fact that one particular sports team had a huge amount of momentum on a particular day might be described as random chance, but that doesn't disprove the claim that it did have momentum then.


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