Think of the last impulse buy you made at the grocery store. Maybe the item was placed at eye level. Or perhaps it was a Snickers bar you saw in the checkout line. Either way, that product was put there by design, not dumb luck, and most of these placements were decided through randomized trials. We talk to with Andrew Leigh, author of “Randomistas: How Radical Researchers Changed Our World,” about how these tests affect everyday life and impact the decisions we make.