'Over the past decade, the power of A/B testing has become an open secret of high-stakes web development. It’s now the standard (but seldom advertised) means through which Silicon Valley improves its online products. Using A/B, new ideas can be essentially focus-group tested in real time: without being told, a fraction of users are diverted to a slightly different version of a given web page and their behaviour compared against the mass of users on the standard site. If the new version proves superior — gaining more clicks, longer visits, more purchases — it will displace the original; if the new version is inferior, it’s quietly phased out without most users ever seeing it. A/B allows seemingly subjective questions of design — colour, layout, image selection, text — to become incontrovertible matters of data-driven social science.'