In the early 1980s, Paul Sackett, a psychologist at the University of Minnesota, began measuring the speed of cashiers at supermarkets. Workers were told to scan a few dozen items as quickly as possible while a scientist timed them. Not surprisingly, some cashiers were much faster than others.

But Mr. Sackett realized that this assessment, which lasted just a few minutes, wasn't the only way to measure cashier performance. Electronic scanners, then new in supermarkets, could...