If you ask Seungjin Whang about the research he and colleague Hau Lee do in supply chain management, he is likely to answer with a story about diapers. Babies consume diapers at a relatively steady rate, he will tell you. Month to month, the number of babies and the number of diapers that their parents purchase from Kmart, Safeway, or the local convenience store remain roughly the same. But several years ago, Procter & Gamble, which manufactures Pampers, studied its diaper sales to retailers and, despite the steady rate of demand among babies, found puzzling, dramatic fluctuations in retailers' orders to wholesalers.