A shift from full-time to part-time work across many major American companies over the past 20 years stems in large part from the rise of what’s known as just-in-time scheduling. Instead of guaranteeing the bulk of their staffs 40 hours a week, many big employers largely reliant on low-wage workers make a majority of them part time and then schedule only the bare minimum number that they expect to need. If customer traffic turns out to be busier than anticipated, those companies have a large pool of part-time workers to call in for last-minute shifts.