# Real life probability quiz

Mark Bowden adapted a piece of his upcoming book about the assassination of Osama Bin Laden for Vanity Fair. It's a crackling read.

What's most fascinating is the insight into how Obama finally made the decision to move on the compound in Abbottabad. As Obama has noted before, he only gets called on to make the tough decisions. The easy ones are made for him.

“One of the things you learn as president is you’re always dealing with probabilities,” he told me. “No issue comes to my desk that is perfectly solvable. No issue comes to my desk where there’s 100 percent confidence that this is the right thing to do. Because if people were absolutely certain then it would have been decided by someone else. And that’s true in dealing with the economic crisis. That’s true in order to take a shot at a pirate. That’s true about most of the decisions I make during the course of the day. So I’m accustomed to people offering me probabilities. In this situation, what you started getting was probabilities that disguised uncertainty as opposed to actually providing you with more useful information.” The president had no trouble facing reality. If he acted on this, he was going to be taking a gamble.

As Nate Silver notes in The Signal and the Noise, one of the reasons more data doesn't always lead to better predictions is that people interpret the data subjectively. The intelligence on Abbottabad was weighted by each member of Obama's administration along with all the evidence they'd accumulated in each of their lives, and so everyone put different likelihoods on whether the man seen pacing in the garden of the compound (nicknamed The Pacer) was actually Bin Laden. The result, Bowden notes, was a distribution of confidence, with most at 80 percent confidence but some as low as 30 percent.

At one meeting, Obama asked Morell, who was seated in a chair against the wall behind him, under the presidential seal, for his own view. Morell put the probability that The Pacer was bin Laden at 60 percent.