The fact that Paul McCartney and John Lennon practiced rock music for 10,000 hours and then became the Beatles does say something about the value of rehearsal and persistence, but that number itself means nothing. I myself have put in that kind of time playing guitar, as have many others whose music you’ll never hear. Whatever it was that allowed Lennon and McCartney to turn practice into genius, it’s unique to them. On the other hand, every number in this book has many hundreds, often many thousands, of people behind it, none of them famous. Here’s the kernel of it: the phrase “one in a million” is at the core of so many wonderful works of art. It means a person so special, so talented, so something that they’re practically unique, and that very rareness makes them significant. But in mathematics, and so with data, and so here in this book, the phrase means just the opposite: 1/1,000,000 is a rounding error.