Brazil entered last Tuesday's World Cup semifinal match missing two of their best players -- Neymar, who was out with an injury, and Silva, who was sitting out a red-card suspension. Would they still be good enough to beat Germany?
After crunching the numbers, Nate Silver, at FiveThirtyEight, forecasted that Brazil still had a 65 percent chance of winning the match -- that the depleted Brazilians were still better than the Germans. In that prediction, he was taking a stand against the betting markets, which actually had Brazil as underdog -- barely -- at 49 percent.
Then, of course, Germany beat the living sh!t out of Brazil, by a score of 7-1.
"Time to eat some crow," Nate wrote after Brazil had been humiliated. "That prediction stunk."
I was surprised; I had expected Nate to defend his forecast. Even in retrospect, you can't say there was necessarily anything wrong with it.
What's the argument that the prediction stunk? Maybe it goes something like this:
-- Defying the oddsmakers, Nate picked Brazil as the favorite.
-- Brazil suffered the worst disaster in World Cup history.
-- Nate's prediction was WAY off.
-- So that has to be a bad prediction, right?
No, it doesn't. It's impossible to know in advance what's going to happen in a soccer game, and, in fact, anything at all could happen. The best anyone can do is try to assign the best possible estimate of the probabilities. Which is what Nate did: he said that there was a 65% chance that Brazil would win, and a 35% chance they would lose.
Nate said Brazil had about twice as much chance of winning as Germany did. He did NOT say that Brazil would play twice as well. He didn't say Brazil would score twice as many goals. He didn't say Brazil would control the ball twice as much of the time. He didn't say the game would be close, or that Brazil wouldn't get blown out.
All he said was, Brazil has twice the probability of winning.
The "65:35" prediction *did* imply that Nate thought Brazil was a better team than Germany. But that's not the same as implying that Brazil would play better this particular game. It happens all the time, in sports, that the better team plays like crap, and loses. That's all built in to the "35 percent".
Here's an analogy.
FIFA is about to pick a random number of dollars between 1 and 1,000,000. I say, there's a 65 percent chance that the number drawn will be higher than the value of a three-bedroom bungalow, which is $350,000.
That's absolutely a true statement, right? 650,000 "winning" balls out of a million is 65 percent. I've made a perfect forecast.
After I make my prediction, FIFA reaches into the urn, pulls out out one of the million balls, and it's ... number 14.
Was my prediction wrong? No, it wasn't. It was exactly, perfectly correct, even in retrospect.
It might SEEM that my prediction was awful, if you don't understand how probability works, or you didn't realize how the balls were numbered, or you didn't understand the question. In that case, you might gleefully assume I'm an idiot. You might say, "Are you kidding me? Phil predicted you could buy a house for $14! Obviously, there's something wrong with his model!"
But, there isn't. I knew all along that there was a chance of "14" coming up, and that factored into my "35 percent" prediction. "14" is, in fact, a surprisingly low outcome, but one that was fully anticipated by the model.
When Nate said that Brazil had a 35 percent chance of losing, a small portion of that 35 percent was the chance of those rare events, like a 7-1 score -- in the same way my own 35 percent chance included the rare event of a really small number getting drawn.
As unintuitive as it sounds, you can't judge Nate's forecast by the score of the game.
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Critics might dispute my analogy by arguing something like this:
"The "14" result in Phil's model doesn't show he was wrong, because, obviously, which ball comes out of the urn it just a random outcome. On the other hand, a soccer game has real people and real strategies, and a true expert would have been able to foresee how Germany would come out so dominating against Brazil."
But ... no. An expert probably *couldn't* know that. That's something that was probably unknowable. For one thing, the betting markets didn't know -- they had the two teams about even. I didn't hear any bettor, soccer expert, sportswriter, or sabermetrician say anything otherwise, like that Germany should be expected to win by multiple goals. That suggests, doesn't it, that it was legimately impossible to foresee?
I say, yes, it was definitely unknowable. You can't predict the outcome of a single game to that extent -- it's a violation of the "speed of light" limit. I would defy you to find any single instance where anyone, with money at stake, seriously predicted a single game outcome that violates conventional wisdom to anything near this extent.
Try it for any sport. On August 22, 2007, the Rangers were 2:3 underdogs on the road against the Orioles. They won 30-3. Did anyone predict that? Did anyone even say the Rangers should be heavy favorites? Is there something wrong with Vegas, that they so obviously misjudged the prowess of the Texas batters?
Of course not. It was just a fluke occurrence, literally unpredictable by human minds. Like, say, 7-1 Germany.
Huh? [Nate Silver] says his prediction “stunk,” but it was probabilistic. No way to know if it was even wrong.
Exactly correct.
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So I don't think you can fault Nate's prediction, here. Actually, that's too weak a statement. I don't mean you have to forgive him, as in, "yeah, he was wrong, but it was a tough one to predict." I don't mean, "well, nobody's perfect." I mean: you have no basis even for *questioning* Nate's prediction, if your only evidence is the outcome of the game. Not as in, "you shouldn't complain unless you can do better," but, as in, "his prediction may well have been right, despite the 7-1 outcome."
But I did a quick Google search for "Brazil 7-1 Nate Silver," and every article I saw that talked about Nate's prediction treated it as certain that his forecast was wrong.
1. Here's one guy who agrees that it's very difficult to predict game results. From there, he concludes that all predictions must therefore be bullsh!t (his word). "Why did they even bother updating their odds for the last three remaining teams at numbers like 64 percent for Germany, 14 percent for the Netherlands, when we just saw how useless those numbers can be?"
Because, of course, the numbers *aren't* bullsh!t, if you correctly interpret them as probabilities and not certainties. If you truly believe that no estimate of odds is useful unless it can successfully call the winner of every game, then how about you bet me every game, taking the Vegas underdog at even money? Then we'll see who's bullsh!tting.
2. This British columnist gets it right, but kind of hides his defense of Nate in a discussion of how sports pundits are bad at predicting. Except that he means that sabermetricians are bad at correctly guessing outcomes. Well, yes, and we know that. But we *are* fairly decent at predicting probabilities, which is all that Nate was trying to do, because he knows that's all that can realistically be done.
"I love Nate Silver and 538, but this result might be breaking his model. Haven't been super impressed with the predictions."
Here's another one:
"To be fair to Nate Silver + 538, their model on the whole was excellent. It's how they dealt with Brazil where I (and others) had problems."
What kind of problems? Not picking them to lose 7-1?
In fairness, sure, there's probably some basis for critiquing Nate's model, since he's been giving Brazil siginficantly higher odds than the bookies. But, in this case, the difference was between 65% and 49%, not between 65% and "OMG, it's a history-making massacre!" So this is not really a convincing argument against Nate's method.
It's kind of like your doctor says, "you should stop smoking, or you're going to die before you're 50!" You refuse, and the day before your fiftieth birthday, a piano falls on your head and kills you. And the doctor says, "See? I was right!"
4. Here's a mathematical one, from a Guardian blogger. He notes that Nate's model assumed goals were independent and Poisson, but, in real life, they're not -- especially when a team collapses and the opponent scores in rapid-fire fashion.
All very true, but that doesn't invalidate Nate's model. Nate didn't try to predict the score -- just the outcome. Whether a team collapses after going down 3-0, or not, doesn't much affect the probability of winning after that, which is why any reasonable model doesn't have to go into that level of detail.
Which is why, actually, losing 7-1 loss isn't necessarily inconsistent with being a favorite. Imagine if God had told the world, "if Brazil falls behind 2-0, they'll collapse and lose 7-1." Nate, would have figured: "Hmmm, OK, so we have to take subtract off the chance of 'Brazil gives up the first two goals, but then dramatically comes back to win the game,' since God says that can't happen."
Nate would have figured that's maybe a 1 percent of all games, and say, "OK, I'm reducing my 65% to 64%."
So, that particular imperfection in the model isn't really a serious flaw.
But, now that I think about it ... imagine that when Nate published his 65% estimate, he explicitly mentioned, "hey, there's still a 1-in-3 chance that Brazil could lose ... and that includes a chance that Germany will kick the crap out of them. So don't get too cocky." That would have helped him, wouldn't it? It might even have made him look really good!
I mean, he shouldn't need to say it to statisticians, because it's an obvious logical consequence of his 65% estimate. But maybe it needs to be said to the public.
"It's hard to imagine how Silver could have been more wrong."
No, it's not hard to imagine at all. If Nate had predicted, "Germany has a 100% chance of winning 7-1," that would have been MUCH more wrong.
First, she argues that Nate should have paid attention to sportswriters, who said Brazil would struggle without those missing players. Researchers need to know when to listen to subject-matter experts, who knew something Nate's mathematical models don't.
Well, first, she's cherry-picking her sportswriters -- they didn't ALL say Brazil would lose badly, did they? You can always find *someone*, after the fact, who bet the right way. So what?
As for subject-matter experts ... Nate actually *is* a subject matter expert -- not on soccer strategy, specfically, but on how sports works mathematically.
On the other hand, a sociology professor is probably an expert in neither. And it shows. At one point, she informs Nate that since the Brazilian team has been subjected to the emotional trauma of losing two important players, Nate shouldn't just sub in the skills of the two new players and run with it as if psychology isn't an issue. He should have *known* that kind of thing makes teams, and statistical models, collapse.
Except that ... it's not true, and subject-matter experts like Nate who study these things know that. There are countless cases of teams who are said to "come together" after a setback and win one for the Gipper -- probably about as many as appear to "collapse". There's no evidence of significant differences at all -- and certainly no evidence that's obvious to a sociologist in an armchair.
Injuries, deaths, suspensions ... those happen all the time. Do teams play worse than expected afterwards? I doubt it. I mean, you can study it, there's no shortage of data. After the deaths of Thurman Munson, Lyman Bostock, Ray Chapman, did their teams collapse? I doubt it. What about other teams that lost stars to red cards? Did they all lose their next game 7-1? Or even 6-2, or 5-3?
Anyway, that's only about one-third of the post ... I'm going to stop, here, but you should read the whole thing. I'm probably being too hard on this professor, who didn't realize that Nate is the expert and not her, and wrote like she was giving a stock lecture to a mediocre undergrad student quoting random regressions, instead of to someone who actually wrote a best-selling book on this very topic.
So, moving along.
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There is one argument that would legitimately provide evidence that Nate was wrong. If any of the critics had chosen to argue convincing evidence for Brazil actually having much less TALENT than Nate and others estimated, evidence that was freely available before the game ... that would certainly be legitimate.
Something like, "Brazil, as a team, is 2.5 goals above replacement with all their players in action, but I can prove that, without Neymar and Silva, they're 1.2 goals *below* replacement!"
That would work.
And, indeed, some of the critiques seem to be actually suggesting that. They imply, *of course* Brazil wouldn't be any good without those players, and how could anyone have expected they would be?
Fine. But, then, why did the bookmakers think they still had a 49% chance? Are you that smart that you saw something? OK, if you have a good argument that shows Brazil should have been 30%, or 20%, then, hey, I'm listening.
If the missing two players dropped Brazil from a 65% talent to a 20% talent, what is each worth individually? Silva is back for today's third-place game against Holland. What's your new estimate for Brazil ... maybe back to 40%?
Well, then, you're bucking the experts again. Brazil is actually the favorite today. The betting markets give them a 62% chance of beating the Netherlands, even though Neymar is still out. Nate has Brazil at 71%. If you think the Brazilians are really that bad, and Nate's model is a failure, I hope you'll be putting a lot of money on the Dutch today.
Because, you can't really argue that Brazil is back to their normal selves today, right? An awful team doesn't improve its talent that much, from 1-7 to favorite, just from the return of a single player, who's not even the team's best. No amount of coaching or psychology can do that.
If you thought Brazil's 7-1 humiliation was because of bad players, you should be interpreting today's odds as a huge mistake by the oddsmakers. I think they're confirmation that Tuesday's outcome was just a fluke.
As I write this, the game has just started. Oh, it's 2-0 Netherlands. Perfect. You're making money, right? Because, if you want to persuade me that you have a good argument that Nate was obviously and egregiously incorrect, now you can prove it: first, show me where you wrote he was still wrong and why; and, second, tell me how much you bet on the underdog Netherlands.
Otherwise, I'm going to assume you're just blowing hot air. Even if Brazil loses again today.
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Update/clarification: I am not trying to defend Nate's methodology against others, and especially not against the Vegas line (which I trust more than Nate's, until there's evidence I shouldn't).
I'm just saying: the 7-1 outcome is NOT, in and of itself, sufficient evidence (or even "good" evidence) that Nate's prediction was wrong.
Labels: 7-1, brazil, forecasting, predictions, soccer
