In mass-production plants, problems tended to be treated as random events. The idea was simply to repair each error and hope that it didn't recur. Ohno instead instituted a system of problem-solving called “the five why’s.” Production workers were taught to trace systematically each error back to its ultimate cause (by asking “why” as each layer of the problem is uncovered), then to devise a fix, so that it would never occur again. > >
Not surprisingly, as Ohno began to experiment with these ideas, his production line stopped all the time, and his workers easily became discouraged. However, as the work teams gained experience identifying and tracing problems back to their root cause, the number of errors began to drop dramatically.
from The Machine That Changed the World, p. 56.
The five whys is an essential tool that I use often, but not often enough. The mistake I often make is believing that asking the questions is my responsibility alone, rather than the responsibility of the whole team.