Local shoppers at the California-based grocery store chain Trader Joe's and Massachusetts’ own Market Basket know there is no self-checkout in either store. Even as self-service checkouts gained floor space in other American grocery and retail stores, those two bet that their customers preferred the efficiency of trained human staff. And recently, some large retailers — Walmart, Costco and Dollar General — have announced they are rethinking self-checkout by adding more staff, or in some cases, removing the machines altogether. Advocates say self-checkout is convenient and reduces employee costs, while critics argue the system has failed to deliver on those promises as many businesses have been hit with merchandise losses from customer errors and shoplifting. Under the Radar looks to the future of self-checkout to find out if the practice has gone stale. GUESTS Rita McGrath, academic director of executive education at Columbia Business School Phil Lempert, founder and CEO of Supermarket Guru, a food and health news hub