When New York City's mayor, Bill de Blasio, went to Albany earlier this week to talk about his program for universal preschool, the discussion reportedly focused on funding, not on whether or how preschool would actually help children. President Obama seemed equally confident when he introduced his plan for universal preschool last year, flatly stating, "We know this works." But the state of research is actually much murkier. And unless policy makers begin to design preschool programs in ways that can be evaluated later, the situation won’t get any clearer. A preschool that "works" could mean different things. It might simply be a safe spot for kids to go. Or it could be a means to get poor kids ready to learn reading and math; they are currently eight to 10 months behind wealthy kids when they start kindergarten. Mayor de Blasio and the president are more ambitious: They think that preschool ought to change life trajectories, resulting in more high school graduates and fewer prison inmates.