Steven Johnson, author of Extra Life, tells the fascinating history of life expectancy, and the extraordinary achievements of the last century, in which it has practically doubled. It’s a story that has data at its heart, from the ground-breaking invention of the category itself in 17th century London to the pioneering social health surveys of W.E.B. Du Bois in 1890s Philadelphia. Tim Harford spoke to Steven about the numbers beneath possibly the most important number of all.