The variables that a production system manipulates to produce value for customers. The first three are resources, the fourth is the way the resources are used.
In a lean system, the Four Ms mean:
- Material—no defects or shortages.
- Machine—no breakdowns, defects, or unplanned stoppages.
- Man—good work habits, necessary skills, punctuality, and no unscheduled absenteeism.
- Method—standardized processes, maintenance, and management.